MBS-4669: Allow removing Amazon ASINs from release_meta

If all ASIN relationships were removed from a release, cache_cover_art
would get called, but because no cover art data was found, the
release_meta row would never be updated.

Now the release_meta row is always updated, and if no artwork could
be found then all the existing fields are cleared.
MBS-4669: Add a fallback to populate release_meta

There are some Amazon ASIN's which are valid, but do not contain
any artwork. If a release has only a relationship to an Amazon ASIN
of this nature, then release_meta should still contain the ASIN, but
not link to any artwork.

The cover art code previously only populated release_meta from the
results of a sucessful cover art lookup - if cover art lookup failed,
then there would be no ASIN to populate release_meta with.

This commit introduces the idea of 'fallback_meta', which is used
to populate release_meta in the scenario that no artwork is found.
MBS-4669: Add a script to update release_meta with correct ASINs

The script uses the same regular expression over the set of all
Amazon ASIN relationships for releases, and if the current
release_meta.amazon_asin value isn't contained in this set, then
that column is updated to be the most recently updated Amazon ASIN
relationship.
Merge branch 'mbs-4669' into beta