Use the polyfiller provided by react-native-webrtc.

Drop the permissions API polyfill since we no longer use it in the app
and lib-jitsi-meet already works without it.

Move all polyfills to a standalone feature, which gets imported before anything
else in the mobile entrypoint. This guarantees that any further import sees the
polyfilled environment.

It causes false positives when submitting the app to the Store. Use the new
permissions API in react-native-webrtc instead.

Recent changes in lib-jitsi-meet probably led to (1) our
RTCPeerConnection customizations on react-native not being used which is
a problem because we need them for at least NAT64 on iOS in order to
pass the review in Apple's App Store and (2) unexpected exceptions
inside react-native-webrtc.

The Promise-based WebRTC API should be merged from react-native-webrtc's
upstream but I don't want to do it right now because last time we got
multiple bugs in addition.

Required for P2P.

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.