Up until now we relied on implicit loading of middlewares and reducers, through
having imports in each feature's index.js.

This leads to many complex import cycles which result in (sometimes) hard to fix
bugs in addition to (often) breaking mobile because a web-only feature gets
imported on mobile too, thanks to the implicit loading.

This PR changes that to make the process explicit. Both middlewares and reducers
are imported in a single place, the app entrypoint. They have been divided into
3 categories: any, web and native, which represent each of the platforms
  respectively.

Ideally no feature should have an index.js exporting actions, action types and
components, but that's a larger ordeal, so this is just the first step in
getting there. In order to both set example and avoid large cycles the app
feature has been refactored to not have an idex.js itself.

Bring back the workaround introduced in afd2aea7
but removed in 21dcc41d. On conference join,
several other actions have already been fired
that try to set the large video participant
and select the participant on the bridge.
The problem is there is no conference during
these actions so the select participant
never fires. Then subsequent actions do not
fire select participant because the large
video participant has not changed.

* ref(large-video): combine selectParticipant logic from web

Currently native/middleware/redux has its own logic for selecting a participant
on the bridge. To have the logic web respect that logic, a few changes are
needed.
- Web no longer has its own call to selectParticipant.
- To keep in line with web logic selectParticipant action should act even when
  there is no track. This makes it so that when a participant does get a track
  that the bridge will send high quality. The bridge can already handle when the
  selected participant does not have a video track.
- The timing of web is such that on joining an existing conference, a
  participant joins and the participant's tracks get updated and then the
  conference is joined. The result is selectParticipant does not get fired
  because it no-ops when there is no conference. To avoid having to make
  uncertain changes (to be lazy), update the selected participant on conference
  join as well.

* squash: update comment, pass message to error handler

In the current middleware logic, when the local participant becomes
dominant speaker, a new participant can be selected to receive
high quality video from. This means large-video could potentially
do a switch to another participant when the local participant
becomes dominant speaker. Prevent such behavior.

The toolbar's mute buttons depict respective features/base/media states.
However, (un)muting is practically carried out by features/base/tracks.
When the mobile app enters a conference configured to invite the joining
participant to mute themselves, the tracks would be muted but the
toolbar's mute buttons would not reflect that.

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.