MBS-6570: Allow clearing relationship dates in relationship editor

If a begin or end date are removed for a relationship, the field doesn't
get submitted at all. To the server code, this is interpreted as
"the user doesn't want to modify this field, whatever it's current contents
are" - which is not what the user intendend.

As a workaround, I am now defaulting undef begin/end dates to an empty
hash, in the code that submits "edit relationship" edits. An empty hash
means unset year/month/day fields.
MBS-1820: Add more information to release search results

Adds release labels, medium formats, release group type and
release status to the display of the search results for releases.
MBS-6591: Show work relationships in JSON web service

/ws/2 has support for showing artist relationships of work
relationships. This commit enables this for the JSON serializer too.
MBS-1820: Remove accidental duplication of code
MBS-1820: Use try_get_session instead of c.session
MBS-6591: Add a test that recording->work->artist relationships show in JSON
MBS-6628: Correctly accept merge label edits with subscriptions

The Edit::Role::MergeSubscription role was incorrectly generalised, and
still had 'artist' hard-coded. This works fine for artist merges, but
fails for label merges as the deleted subscriptions have the 'label' key,
rather than 'artist'. This commit changes this code to call model_to_type
to identify the key name.
Merge branch 'mbs-6628' into beta
Merge branch 'mbs-1820' into beta
Merge branch 'mbs-6591' into beta
Merge branch 'mbs-6570' into beta

Conflicts:
	postgresql-musicbrainz-collate
	postgresql-musicbrainz-unaccent