See multiple participants on stage
Pin and unpin to stage
Automatic selection of participants to be displayed on the stage filmstrip based on dominant speaker changes
Make Filmstrip a reusable component. Used by MainFilmstrip (old functionality) and the new StageFilmstrip
Rename DominantSpeakerName to StageParticipantNameLabel
Active border now showed only for the dominant speaker (no longer for the pinned participant)
Hide video from the vertical filmstrip for the participants on stage
Update video constraints
Updated pinned indicator

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.