Previous code was not checking/enforcing rule 4.3.1
(https://spec.matrix.org/v1.2/rooms/v9/), which states that a room
member event (with a join membership) must be accepted only if it contains
a previous event which `state_key` is the room creator.
On top of that, it simplifies the public interface for `auth_rules`,
removing the need of (externally) compute `prev_event`, which, as a side
effect, should reduce the amount of times the previous event needs to be
fetched: It will only load it if the authorized event is a `m.room_member`
with a `Join` state.
Finally, I've splitted the join conditions so it's (hopefully) more
readable and apply auth rules in the same order as they
appear in the spec.
fca0f6a22b