Automatic album artwork download

This PR adds an automatic album, artwork downloader, that fetches artworks from the MusicBrainz API

- [x] Create the Musicbrainz fetcher
- [x] Let the ArtworkCache handle everything intelligently
- [x] Fire the search off after the first library update for artists that have no cover yet
- [x] Make sure we don't apply the wrong artwork (MusicBrainz has a score in the search API that indicates how likely the search matches)
- [x] If the first release lookup doesn't contain an artwork, try the next, as long as the score is over a threshold