MBS-13421: Add support for genre collections

[reosarevok]

Implement MBS-13421

Description

This adds support for collections of genres, in the same way we already do for all other sorts of collections. Nothing that I can remember needed to be been special-cased for genre, and since genre has less data than other entities it doesn't even load any extra info for display like other entities.

Testing

I manually ran the schema change files, ensured that I can add, see, remove and merge genre collections, that they show on the genre page sidebar, that adding and removing genres from a collection works from the sidebar, that genre collections are listed on the WS /collections endpoint and that I can browse collections using a genre MBID.

Extended a couple basic WS tests to also check genre collections (and by not adding anything to them, also now test empty collections).

Documenting

• [ ] A genre row including collection needs to be added to https://musicbrainz.org/doc/MusicBrainz_API/#Linked_entities

Deployment

This is coupled with mmd-schema PR https://github.com/metabrainz/mmd-schema/pull/37.

[mwiencek]

Now that https://github.com/metabrainz/mmd-schema/pull/37 was merged, in order to get tests working here we'll need to update the mmd-schema submodule in mb-solr, tag a new release, bump MB_SOLR_TAG in the tests Dockerfile, and rebuild the tests image.