During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
To track changes in MSC2666:
- The change from `/mutual_rooms/{user_id}` to `/mutual_rooms?user_id={user_id}`.
- The addition of `next_batch_token` (and logic).
- Unstable flag now being `uk.half-shot.msc2666.query_mutual_rooms`.
- The error code when your own user is requested.
