During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
* Change `create_room` return type
* Don't return room alias from /createRoom
* Update other callsites
* Fix up mypy complaints
It looks like new_room_user_id is None iff new_room_id is None. It's a
shame we haven't expressed this in a way that mypy can understand.
* Changelog
This adds support for the stable identifiers of MSC2285 while
continuing to support the unstable identifiers behind the configuration
flag. These will be removed in a future version.
