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.

There is no need for AbstractApp to require some getWindowLocation function.
It's only used in one place and we even polyfill it on mobile.

Thus replace it's usage with more specific functions.

BaseApp does all the heavy-lifting related to creating the redux store,
navigation, and so on.

App currently handles URL props and actually triggering navigation based on
them.

Move it away from AbstractApp into an auxiliary function. In addition, introduce
a new `getServerURL` function which gets the configured server URL and defaults
to meet.jit.si as before.

If multiple JitsiMeetView instances are created (not necessarily
existing at once), it's possible to hit a TypeError when reading the
React Component props of the currently mounted App. Anyway, in certain
places we're already protecting against that out of abundance of caution
so it makes no sense to not protect everywhere.