Store track artist and album artist separately (#237)

Better handling of tracks with a different artist than the album artist
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Some tracks involve a different artist than the album artist (e.g. a featuring)
and Funkwhale has been known to do weird things when importing such tracks, resulting
in albums that contained a single track, for instance.

The situation should be improved with this release, as Funkwhale is now able to
store separately the track and album artist, and display it properly in the interface.