Previously, the CallView was attached to the RoomView, so you would get
a new CallView each time you changed the room and the one you changed
from would be destroyed. This would destroy media capture/playback as
the element was no longer in the DOM.
This is now fixed by having a "global" CallView which is attached at
the MatrixChat "page" level in the DOM hierarchy. This CallView isn't
scoped to a particular room; it will render any "active" call it can
find that *isn't the current room being displayed*. This has the side
effect of enforcing 1 call per app semantics as only the first active
call found is returned.
This fixes https://github.com/vector-im/vector-web/issues/31
This is unfinished (CSS for the global call view isn't done)
59986d8b72