Prepare for React 16.

https://github.com/jitsi/jitsi-meet/pull/1397 (React Toolbar) is huge at
the time of this writing. In order to reduce it, I'm extracting changes
not directly related to React-ifying the Toolbar such as added jsdocs
and source code formatting.

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.