Introduce navigation for all in-conference screens.

* 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>

As an intermediate step on the path to merging jitsi-meet and
jitsi-meet-react, import the whole source code of jitsi-meet-react as it
stands at
2f23d98424
i.e. the lastest master at the time of this import. No modifications are
applied to the imported source code in order to preserve a complete
snapshot of it in the repository of jitsi-meet and, thus, facilitate
comparison later on. Consequently, the source code of jitsi-meet and/or
jitsi-meet-react may not work. For example, jitsi-meet's jshint may be
unable to parse jitsi-meet-react's source code.