We now fetch album covers regardless of the import methods (#231)

Smarter album cover importer
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In earlier versions, covers where only imported when launching a YouTube import.
Starting from this release, covers will be imported regardless of the import mode
(file upload, youtube-dl, CLI, in-place...). Funkwhale will look for covers
in the following order:

1. In the imported file itself (FLAC/MP3 only)
2. In a cover.jpg or cover.png in the file directory
3. By fetching cover art from Musibrainz, assuming the file is tagged correctly

This will only work for newly imported tracks and albums though. In the future,
we may offer an option to refetch album covers from the interface, but in the
meantime, you can use the following snippet:

.. code-block:: python

    # Store this in /tmp/update_albums.py
    from funkwhale_api.music.models import Album, TrackFile
    from funkwhale_api.music.tasks import update_album_cover

    albums_without_covers = Album.objects.filter(cover='')
    total = albums_without_covers.count()
    print('Found {} albums without cover'.format(total))
    for i, album in enumerate(albums_without_covers.iterator()):
        print('[{}/{}] Fetching cover for {}...'.format(i+1, total, album.title))
        f = TrackFile.objects.filter(track__album=album).filter(source__startswith='file://').first()
        update_album_cover(album, track_file=f)

Then launch it::

    # docker setups
    cat /tmp/update_albums.py | docker-compose run --rm api python manage.py shell -i python

    # non-docker setups
    source /srv/funkwhale/load_env
    source /srv/funkwhale/virtualenv/bin/activate
    cat /tmp/update_albums.py | python manage.py shell -i python

    # cleanup
    rm /tmp/update_albums.py

.. note::

    Depending on your number of albums, the previous snippet may take some time
    to execute. You can interrupt it at any time using ctrl-c and relaunch it later,
    as it's idempotent.