Merge AlbumTrack and Media

In order to solve #372 (closed) we'll need to merge the AlbumTrack & Media tables.

This means:

Adding the artist_id, genre_id, track_number, album_id, disc_number fields to the media table
Triple checking uses of media.duration, since the album track field was guaranteed to be >= 0 while the media one is not
Dropping the AlbumTrack class

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