There is a race condition in the root navigatior's initialization.

It's possible that it's initialized a touch too late and SDK users who
try to navigate to a conference end up stuck in the connecting screen
because the navigator is null.

This PR waits for it to be initilized by very unorthodox means, it's a
horrible hack which we need to undo, but for that we need to break
appart the inheritance relationship between App.{web,native},
AbstractApp and BaseApp because it's very inflexible.

The flags are now initialized very early so the naviggator sees if the
welcome page is enabled or not.

This "feature" has been dead (and most likely buggy) for years. The
recommended way is for apps to implement their own incoming call
handling and then call into the JitsiMeetActivity. We did not have those
APIs back then.

Also unify the mobile and web features into one, even though internally they still have separate ways to enable the functionality.

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.