Adds base/sounds feature which allows other features to register a sound
source under specified id. A new SoundsCollection component will then
render corresponding HTMLAudioElement for each such sound. Once "setRef"
callback is called by the HTMLAudioElement, this element will be added
to the Redux store. When that happens sound can be played through the
new 'playSound' action which will call play() method on the stored
HTMLAudioElement instance.

Android uses a SurfaceView to render video, which is not quite a View, so the
fade-in animation (which varies the opacity) doesn't work.

Instead, add an opaque black view covering the video, which transitions to
transparent. This creates much smoother transitions on Android, while behaving
the same.

In addition, I removed the flip animation for local tracks, which is no longer
used, since the camera is switched without changing tracks.

Prepare for React 16.

The files styles.js are used (pretty much) on React Native only and each
of them exports 1 name. Export it as default to cut down on source code.

Introduce certain React Components which may be used to write
cross-platform source code such as Audio like Web's audio, Container
like Web's div, Text like Web's p, etc.

It's now natively supported:
https://github.com/oney/react-native-webrtc/pull/244

Until we make a decision on access modifier hints and adopt a respective
coding style, consistency is king.

React Native provides Platform.

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.