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

@@ -13,6 +13,7 @@ import { CalleeInfoContainer } from '../../invite';
 import { LargeVideo } from '../../large-video';
 import { NotificationsContainer } from '../../notifications';
 import { SidePanel } from '../../side-panel';
+import { default as Notice } from './Notice';
 import {
     Toolbox,
     fullScreenChanged,
@@ -163,6 +164,7 @@ class Conference extends Component<Props> {
             <div
                 id = 'videoconference_page'
                 onMouseMove = { this._onShowToolbar }>
+                <Notice />
                 <div id = 'videospace'>
                     <LargeVideo
                         hideVideoQualityLabel = { hideVideoQualityLabel } />