User authentication with MetaBrainz OAuth

[kellnerd]

1. Enables tag submission (Add Genre / Tag support #141).
2. Requiring users to be logged in would help to lock out random bots and crawlers, leaving more resources (both Harmony server resources and upstream API quota) for actual users. I don't think this is unreasonable to ask, every user needs a MetaBrainz account anyway in order to import into MusicBrainz.
3. ISRC submission without third party tools could also be implemented (if someone wants to streamline that process).