handlers/_base: don't allow room create event to be changed

[Rugvip]

Beginning of the m.room.create spec says

This is the first event in a room and cannot be changed.

This could arguably be interpreted as that specific event not being changed, or the m.room.create state as a whole. I chose to interpret it as the latter, which I also think makes sense, as it is currently possible to e.g. change the creator key of the create state.

In either case this is a spec bug as well.

[matrixbot]

Can one of the admins verify this patch?

[erikjohnston]

@matrixbot ok to test