Because users can change their username in musicbrainz, we should use a more stable identifier instead of the username for users in acousticbrainz

https://github.com/metabrainz/acousticbrainz-server/blob/f9afa4017c49b7d412b6557b68744d8fbbb2da07/db/user.py#L9-L16

This was already implemented in ListenBrainz: https://github.com/metabrainz/listenbrainz-server/pull/415