noticeMessage is not shown (refs #3295)

* Get back the Notice class
* Add Notice component in the Conference web view
* Notice is not exported in index.js. Only used internally by
  Conference.
* noticeMessage value obtained from features/base/config
  * using mapStateToProps
  * value is stored in the internal _message property
* Notice component, orignal in `toolbox` is moved from
  `toolbox/components` to `conference/components`
* Notice component only implemented and renderable in web views
* Dummy `conference/components/Notice.naive.js`

This patch is partially based in the removed logic included
originally in:

    commit 59a74153dc
    (tag: jitsi-meet_1886, tag: jitsi-meet_1885, tag: 1797, tag: 1796)
    Author: Ilya Daynatovich <shupuercha@gmail.com>
    Date:   Mon Mar 20 11:04:54 2017 -0500

      Toolbar notice as React Component

In reply to: Saúl Ibarra Corretgé @saghul> comments

Signed-off-by: Pablo Saavedra <psaavedra@igalia.com>

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

 import { LargeVideo } from '../../large-video';
 import { NotificationsContainer } from '../../notifications';
 import { SidePanel } from '../../side-panel';
+import { default as Notice } from './Notice';
 import {
     Toolbox,
     fullScreenChanged,
             <div
                 id = 'videoconference_page'
                 onMouseMove = { this._onShowToolbar }>
+                <Notice />
                 <div id = 'videospace'>
                     <LargeVideo
                         hideVideoQualityLabel = { hideVideoQualityLabel } />

react/features/conference/components/Notice.web.js

+/* @flow */
+
+import React, { Component } from 'react';
+import { connect } from 'react-redux';
+
+import { translate } from '../../base/i18n';
+
+declare var config: Object;
+
+type Props = {
+    _message?: string,
+};
+
+/**
+ * Notice react component.
+ *
+ * @class Notice
+ */
+class Notice extends Component<Props> {
+
+    /**
+     * Implements React's {@link Component#render()}.
+     *
+     * @inheritdoc
+     * @returns {ReactElement}
+     */
+    render() {
+        if (!this.props._message) {
+            return null;
+        }
+
+        return (
+            <div className = 'notice'>
+                <span className = 'notice__message' >
+                    { this.props._message }
+                </span>
+            </div>
+        );
+    }
+}
+
+/**
+ * Maps (parts of) the Redux state to the associated
+ * {@code Notice}'s props.
+ *
+ * @param {Object} state - The Redux state.
+ * @private
+ * @returns {{
+ *     _message: string,
+ * }}
+ */
+function _mapStateToProps(state) {
+    const {
+        noticeMessage
+    } = state['features/base/config'];
+
+    return {
+        _message: noticeMessage
+    };
+}
+export default translate(connect(_mapStateToProps)(Notice));

react/features/conference/components/index.js

+// @flow
+
 export { default as Conference } from './Conference';