Import trust source

Rewrite of the import glue to be less DB and network intensive:

• Always trust tags from the file -> no more discrepencies between what the user expect and what is imported
• No more API calls to musicbrainz to fetch music data -> Faster import. We still continue to store Musicbrainz data though

Todo:

• Use the same code for federation imports, to have even less bugs!
• Import covers from federation

This fix #361 (closed) and #367 (closed)