Previously, when initiating a call by calling multiple people, only
the first person would get the call while all the others would briefly
show a "pending" status but never get the call.
This would happen because `ActiveCall` was trying to a create a different
room for each person called, because the original room creation hadn't finished
and so a `ModelHandle<Room>` wasn't being store in the active call.
With this commit, only one room can be created at any given time and further
invites have to wait until that room creation is done.
