Converted LoadConfigOverlay to a JitsiScreen component that right now is part of navigation as ConnectingPage, plus I
separated appNative and other actions into web and native.

This event is the event host applications need to listen to for knowing when to
dispose the SDK from now on.

Since the introduction of breakout rooms it's possible that we navigate from one
meeting to another, so there will be several conference join / terminations.

In addition, local track destruction is now moved to SET_ROOM when there is no
room, aka, we are going back to the welcome page or to the black page.

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.