Check room alias isn't taken when creating new room

[erikjohnston]

This is slightly tricky as it interacts with #267. We'll probably want to have an API that allows us to "reserve" an alias and later "claim" it once the room is actually created, or something

[spaetz]

just checked whether this is still relevant, and yes as hinted by https://github.com/matrix-org/dendrite/blob/5424b88f3061dddbeae53df8b67bd064e8cbc400/clientapi/routing/createroom.go#L473 we first create a room and then try to create the alias, bailing out when it is already taken. So this is issue is still relevant.