feat: private message interface config flag

react/features/base/config/interfaceConfigWhitelist.js

     'DEFAULT_REMOTE_DISPLAY_NAME',
     'DISABLE_DOMINANT_SPEAKER_INDICATOR',
     'DISABLE_FOCUS_INDICATOR',
+    'DISABLE_PRIVATE_MESSAGES',
     'DISABLE_RINGING',
     'DISABLE_TRANSCRIPTION_SUBTITLES',
     'DISABLE_VIDEO_BACKGROUND',

react/features/remote-video-menu/components/web/PrivateMessageMenuButton.js

 import { translate } from '../../../base/i18n';
 import { IconMessage } from '../../../base/icons';
 import { connect } from '../../../base/redux';
-import { _mapDispatchToProps, _mapStateToProps, type Props } from '../../../chat/components/PrivateMessageButton';
+import {
+    _mapDispatchToProps,
+    _mapStateToProps as _abstractMapStateToProps,
+    type Props as AbstractProps
+} from '../../../chat/components/PrivateMessageButton';
+import { isButtonEnabled } from '../../../toolbox';
 
 import RemoteVideoMenuButton from './RemoteVideoMenuButton';
 
+declare var interfaceConfig: Object;
+
+type Props = AbstractProps & {
+
+    /**
+     * True if the private chat functionality is disabled, hence the button is not visible.
+     */
+    _hidden: boolean
+};
+
 /**
  * A custom implementation of the PrivateMessageButton specialized for
  * the web version of the remote video menu. When the web platform starts to use
      * @returns {ReactElement}
      */
     render() {
-        const { participantID, t } = this.props;
+        const { participantID, t, _hidden } = this.props;
+
+        if (_hidden) {
+            return null;
+        }
 
         return (
             <RemoteVideoMenuButton
     }
 }
 
+/**
+ * Maps part of the Redux store to the props of this component.
+ *
+ * @param {Object} state - The Redux state.
+ * @param {Props} ownProps - The own props of the component.
+ * @returns {Props}
+ */
+function _mapStateToProps(state: Object, ownProps: Props): $Shape<Props> {
+    return {
+        ..._abstractMapStateToProps(state, ownProps),
+        _hidden: typeof interfaceConfig !== 'undefined'
+            && (interfaceConfig.DISABLE_PRIVATE_MESSAGES || !isButtonEnabled('chat'))
+    };
+}
+
 export default translate(connect(_mapStateToProps, _mapDispatchToProps)(PrivateMessageMenuButton));