Fix inconsistencies in event validation for `m.room.create` events (#13087)

* Extend the auth rule checks for `m.room.create` events

... and move them up to the top of the function. Since the no auth_events are
allowed for m.room.create events, we may as well get the m.room.create event
checks out of the way first.

* Add a test for create events with prev_events