renameUser(userId: string,username: string): User
Renames user userId to username, for PATCH /api/users/:key/username,
which only user:manage may call: users can't rename themselves. Returns
them renamed. The old name becomes an alias of theirs for good, so
/@old/… still reaches them and nobody else may take it; a change of case
alone leaves none, since names are looked up in any case. Taking back a
former name of their own is allowed. Their dumps' and playlists' slugs
belong to their id, so their paths follow the new name.