PICARD-418: fallback to using release group cover

It adds the option to use release group cover art from CAA.
It is required to enable CAA and release group fallback is disabled by default

http://tickets.musicbrainz.org/browse/PICARD-418
http://tickets.musicbrainz.org/browse/PICARD-53
Yes, although undocumented, [the tests](https://github.com/metabrainz/coverart_redirect/blob/master/test/test_all.py#L180) in the coverart-redirect cover thumbnails.