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.

- opens as a modal
- lastn is 0, mutes local video while open
- long press to talk
- and more

Only havee the screen in the hierarchy if we have a welcome page, since
it's the only way to access it.

Use goBack() from the navigator directly and avoid duplicating all props
to the screen.

It allows us to place the Cancel button there, and declutter the UI of
buttons.

Skip the welcome page for now, until we can remove the audio / video
toggle.

Update Android build to support gif
Use GIF format instead of animated webp
Show GIFs in chat messages
Display GIF over tile
Add Giphy button in reactions menu
Added Giphy dialog
Fix isGifMessage to also allow upper case

It will just flash for a split second and all it does is dispatch
readyToClose, so do it directly instead.

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.

Updated screens with new ui on mobile

The CONFERENCE_WILL_LEAVE reducer in base/conference wipes the state so
we cannot rely on the old room value.

We may want to revisit this in the future.