Fix eslint & flow errors

Prepares for the latest eslint & flow-related npm packages which cause
these errors.

modules/UI/etherpad/Etherpad.js

     }
 
     /**
-     * 
+     *
      */
     isVisible() {
         return VideoLayout.isLargeContainerTypeVisible(ETHERPAD_CONTAINER_TYPE);

modules/UI/videolayout/VideoContainer.js

     }
 
     /**
-     * 
+     *
      */
     hide() {
         // as the container is hidden/replaced by another container

modules/devices/mediaDeviceHelper.js

         }
 
         /**
-         * 
+         *
          */
         function createVideoTrack(showError) {
             return (

modules/translation/translation.js

     }
 
     /**
-     * 
+     *
      */
     translateElement(selector: Object, options: Object) {
         // XXX i18next expects undefined if options are missing.

react/features/app/components/AbstractApp.js

+/* global APP */
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { I18nextProvider } from 'react-i18next';
 
 import { appNavigate, appWillMount, appWillUnmount } from '../actions';
 
-declare var APP: Object;
-
 /**
  * The default URL to open if no other was specified to {@code AbstractApp}
  * via props.

react/features/base/dialog/components/Dialog.native.js

+// @flow
+
 import PropTypes from 'prop-types';
 import React from 'react';
 import { Modal, StyleSheet, TextInput } from 'react-native';
             <Prompt
                 cancelButtonTextStyle = { cancelButtonTextStyle }
                 cancelText = { t(cancelTitleKey) }
+
+                // $FlowFixMeState
                 defaultValue = { this.state.text }
                 onCancel = { this._onCancel }
                 onChangeText = { this._onChangeText }
                 mapped
                     = React.cloneElement(
                         mapped,
-                        /* props */ undefined,
+                        /* props */ {},
                         ...React.Children.toArray(React.Children.map(
                             children,
                             function(element) { // eslint-disable-line no-shadow
         return mapped;
     }
 
+    _onCancel: () => void;
+
     _onChangeText: (string) => void;
 
     /**
         this.setState({ text });
     }
 
+    _onSubmit: (?string) => void;
+
     /**
      * Submits this {@code Dialog} with the value of the {@link TextInput}
      * rendered by {@link Prompt} unless a value is explicitly specified.
      * @returns {void}
      */
     _onSubmit(value: ?string) {
+        // $FlowFixMeState
         super._onSubmit(value || this.state.text);
     }
 }

react/features/base/media/components/AbstractAudio.js

+// @flow
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 
  * The React {@link Component} which is similar to Web's
  * {@code HTMLAudioElement}.
  */
-export default class AbstractAudio extends Component {
+export default class AbstractAudio extends Component<*> {
     /**
      * The (reference to the) {@link ReactElement} which actually implements
      * this {@code AbstractAudio}.

react/features/conference/components/Conference.native.js

          * @returns {void}
          * @private
          */
-        _setToolboxVisible(visible: boolean) {
+        _setToolboxVisible(visible) {
             dispatch(setToolboxVisible(visible));
         }
     };

react/features/connection-indicator/components/ConnectionIndicator.js

+/* global interfaceConfig */
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 
 
 import statsEmitter from '../statsEmitter';
 
-declare var interfaceConfig: Object;
-
 /**
  * The connection quality percentage that must be reached to be considered of
  * good quality and can result in the connection indicator being hidden.

react/features/device-selection/DeviceSelectionPopup.js

+/* global JitsiMeetJS */
+
 import Logger from 'jitsi-meet-logger';
 import React from 'react';
 import ReactDOM from 'react-dom';
 
 import DeviceSelectionDialogBase from './components/DeviceSelectionDialogBase';
 
-declare var JitsiMeetJS: Object;
-
 const logger = Logger.getLogger(__filename);
 
 /**

react/features/device-selection/actions.js

  * @param {Function} responseCallback - The callback that will send the
  * response.
  * @returns {boolean}
- */ // eslint-disable-next-line max-params
-function _processRequest(dispatch, getState, request, responseCallback) {
+ */
+function _processRequest(dispatch, getState, request, responseCallback) { // eslint-disable-line max-len, max-params
     if (request.type === 'devices') {
         switch (request.name) {
         case 'isDeviceListAvailable':

react/features/feedback/components/FeedbackDialog.web.js

+/* global interfaceConfig */
+
 import StarIcon from '@atlaskit/icon/glyph/star';
 import StarFilledIcon from '@atlaskit/icon/glyph/star-filled';
 import PropTypes from 'prop-types';
 
 import { cancelFeedback, submitFeedback } from '../actions';
 
-declare var interfaceConfig: Object;
-
 const scoreAnimationClass
     = interfaceConfig.ENABLE_FEEDBACK_ANIMATION ? 'shake-rotate' : '';
 

react/features/invite/components/InfoDialog.web.js

+/* global interfaceConfig */
+
 import React, { Component } from 'react';
 import { connect } from 'react-redux';
 import PropTypes from 'prop-types';
 
 const logger = require('jitsi-meet-logger').getLogger(__filename);
 
-declare var interfaceConfig: Object;
-
 /**
  * A React Component with the contents for a dialog that shows information about
  * the current conference and provides ways to invite other participants.

react/features/invite/components/InfoDialogButton.web.js

+/* global interfaceConfig */
+
 import InlineDialog from '@atlaskit/inline-dialog';
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { ToolbarButton, TOOLTIP_TO_POPUP_POSITION } from '../../toolbox';
 
 import { setInfoDialogVisibility } from '../actions';
-
 import InfoDialog from './InfoDialog';
 
-declare var interfaceConfig: Object;
-
 const { INITIAL_TOOLBAR_TIMEOUT } = interfaceConfig;
 
 /**

react/features/invite/components/InviteButton.web.js

+/* global interfaceConfig */
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { connect } from 'react-redux';
 import { DialOutDialog } from '../../dial-out';
 import { isInviteOptionEnabled, getInviteOptionPosition } from '../functions';
 
-declare var interfaceConfig: Object;
-
 const SHARE_LINK_OPTION = 'invite';
 const DIAL_OUT_OPTION = 'dialout';
 const ADD_TO_CALL_OPTION = 'addtocall';

react/features/notifications/components/Notification.web.js

+// @flow
+
 import Flag from '@atlaskit/flag';
 import EditorInfoIcon from '@atlaskit/icon/glyph/editor/info';
-import PropTypes from 'prop-types';
 import ErrorIcon from '@atlaskit/icon/glyph/error';
 import WarningIcon from '@atlaskit/icon/glyph/warning';
 import { colors } from '@atlaskit/theme';
+import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 
 import { translate } from '../../base/i18n';
  *
  * @extends Component
  */
-class Notification extends Component {
+class Notification extends Component<*> {
     /**
      * Default values for {@code Notification} component's properties.
      *
         );
     }
 
+    _onDismissed: () => void;
+
     /**
      * Calls back into {@code FlagGroup} to dismiss the notification.
      *

react/features/overlay/components/OverlayContainer.js

+/* global interfaceConfig */
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { connect } from 'react-redux';
     from './UserMediaPermissionsFilmstripOnlyOverlay';
 import UserMediaPermissionsOverlay from './UserMediaPermissionsOverlay';
 
-declare var interfaceConfig: Object;
-
 /**
  * Implements a React Component that will display the correct overlay when
  * needed.

react/features/overlay/components/OverlayFrame.js

+/* global interfaceConfig */
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 
-declare var interfaceConfig: Object;
-
 /**
  * Implements a React Component for the frame of the overlays.
  */

react/features/overlay/components/ReloadTimer.js

+/* global AJS */
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 
 import { translate } from '../../base/i18n';
 
-declare var AJS: Object;
-
 /**
  * Implements a React Component for the reload timer. Starts counter from
  * props.start, adds props.step to the current value on every props.interval

react/features/remote-control/components/RemoteControlAuthorizationDialog.js

+// @flow
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { connect } from 'react-redux';
 /**
  * Implements a dialog for remote control authorization.
  */
-class RemoteControlAuthorizationDialog extends Component {
+class RemoteControlAuthorizationDialog extends Component<*> {
     /**
      * RemoteControlAuthorizationDialog component's property types.
      *
         );
     }
 
+    _onCancel: () => boolean;
+
     /**
      * Notifies the remote control module about the denial of the remote control
      * request.
         return true;
     }
 
+    _onSubmit: () => boolean;
+
     /**
      * Notifies the remote control module that the remote control request is
      * accepted.

react/features/remote-video-menu/components/RemoteVideoMenuTriggerButton.js

+// @flow
+
 import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 
  *
  * @extends {Component}
  */
-class RemoteVideoMenuTriggerButton extends Component {
+class RemoteVideoMenuTriggerButton extends Component<*> {
     static propTypes = {
         /**
          * A value between 0 and 1 indicating the volume of the participant's
         remoteControlState: PropTypes.number
     };
 
+    /**
+     * The internal reference to topmost DOM/HTML element backing the React
+     * {@code Component}. Accessed directly for associating an element as
+     * the trigger for a popover.
+     *
+     * @private
+     * @type {HTMLDivElement}
+     */
+    _rootElement = null;
+
     /**
      * Initializes a new {#@code RemoteVideoMenuTriggerButton} instance.
      *
      * @param {Object} props - The read-only properties with which the new
      * instance is to be initialized.
      */
-    constructor(props) {
+    constructor(props: Object) {
         super(props);
 
-        /**
-         * The internal reference to topmost DOM/HTML element backing the React
-         * {@code Component}. Accessed directly for associating an element as
-         * the trigger for a popover.
-         *
-         * @private
-         * @type {HTMLDivElement}
-         */
-        this._rootElement = null;
-
         // Bind event handler so it is only bound once for every instance.
         this._onShowRemoteMenu = this._onShowRemoteMenu.bind(this);
     }
         );
     }
 
+    _onShowRemoteMenu: () => void;
+
     /**
      * Opens the {@code RemoteVideoMenu}.
      *