SEARCH-681 (indexer): Index genres

[reosarevok]

Implement SEARCH-681

AFAICT this is all that needs to be changed, except for generating new triggers and whatnot so that they check genre_annotation. But I didn't manage to generate them locally with the command in a8798bc - dunno if that's changed or something is missing here (it complains about ModuleNotFoundError: No module named 'sentry_sdk' but I also can't run setup properly, it seems, because then it complains about No such file or directory: 'mbrng/init.py). Maybe someone else can generate them and squash?

[yvanzo]

The code changes look good so far but I didn’t test it either.

The reindex couldn’t work because of a bug in the commit dff0a11 it was based on. A fix for this bug is in master but it depends on code migrated to SQLAlchemy 1.4 and which isn’t production-ready yet. So as a workaround, this pull request can be based on the temporary branch released-master (which match the latest release v3.0.1) until master gets stabilized.

It requires that the genre core exists. This core will be defined by running a new release of mb-solr that makes use of metabrainz/mbsssss#61. SIR reindex will fail without that.

Note that the latest release v-2020-11-12 of mmmd-schema doesn’t have genre’s annotation, aliases, and relationships. So the upcoming release of mb-solr should also use a new release of mmd-schema to fully implement SEARCH-680 and SEARCH-681 at once.