This PR adds an additional `room_config` argument to the
`user_may_create_room` spam checker module API callback.
It will continue to work with implementations of `user_may_create_room`
that do not expect the additional parameter.
A side affect is that on a room upgrade the spam checker callback is
called *after* doing some work to calculate the state rather than
before. However, I hope that this is acceptable given the relative
infrequency of room upgrades.
