Display spinner not room preview after room create
After creating a room, display the activity spinner while we wait
for the room to come down the event stream.
This was the intention before but I can't see how it would have
worked: we were setting the 'joining' flag, but then resetting it
by claiming we were already joined in the view_room dispatch.
* Send 'joining' instead of 'joined' in view_room dispatch, which
will set the corresponding joining flag (ie. to indicate we've
sent a request to join the room). Remove the 'joined' flag.
* Reset 'joining' to false otherwise on a view_room dispatch to
prevent it from leaking between rooms (this may have been the
intention of the `if (payload.joined) newState.joining = false`?
Fixes https://github.com/vector-im/riot-web/issues/4701
