ElementCallView took muteStates as a prop, which would have meant an
embedder building one before it could show a call. It could not simply make
its own: RoomPage created one on mount whichever branch it went on to
render, and two would both report the user's mute state to the host,
talking over each other.
Move the construction into a useMuteStates hook, and give the lobby shown
while waiting to be let into a room its own component. Each lobby now holds
mute state only while it is on screen, so there is never a second one, and
the component can own the call's.
KnockLobbyView also takes the room summary and label handling that RoomPage
was assembling on its behalf, leaving the page with arriving at a call
rather than being in one.
