* feat: Use source name based receiver constraints

* add sourceNameSignaling feature flag to config

* move source name specific variables into if block

* ensure sourceName is defined in constraints

* use source name for selectedSources

* use selector to find video track by participant id

* add selector to encapsulate logic to get track source name

* refactor getTrackSourceNameByMediaTypeAndParticipant selector

* rename variable

* move flags config into Feature Flags section

* do not set constraints for local large video

* rename prioritizedSources to onStageSources

* fix flow error

* refactor visibleRemoteTrackSourceNames to a constant

* use selector to get feature flags

* rename selector function

* fix flow error

* add selector for sourceNameSignaling feature flag

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.

The config object defined by lib-jitsi-meet is not used by
lib-jitsi-meet only, jitsi-meet respects its values as well.
Moreover, jitsi-meet defined classes and/or functions which manipulate
that config object. Consequently, it makes sense to move the config
object and the associated classes and functions in a dedicated feature.

The Device Selection modal consists of:
- DeviceSelection, an overly smart component responsible for
  triggering stream creation and cleanup.
- DeviceSelector for selector elements.
- VideoInputPreview for displaying a video preview.
- AudioInputPreview for displaying a volume meter.
- AudioOutputPreview for a test sound output link.

Store changes include is primarily storing the list of
available devices in redux. Other app state has been left
alone for future refactoring.

On Android the status and navigation bars are shown again after coming back from
the background, so enter full-screen mode again if needed.

The implementation varies across platforms, with the same goal: allow the app to
use the entire screen real state while in a conference.

On Android we use immersive mode, which  will hide the status and navigation bars.

https://developer.android.com/training/system-ui/immersive.html

On iOS the status bar is hidden, with a slide effect.