Handle reviews with deleted entities in musicbrainz-db

What?

Currently, the deployed code on beta.critiquebrainz.org include merges from the following PRs:

https://github.com/metabrainz/brainzutils-python/pull/29

https://github.com/metabrainz/critiquebrainz/pull/262

The changes in CB also include using brainzutils.musicbrainz_db.utils.get_entities_by_gid. In CB, the implemented function from critiquebrainz.musicbrainz_db.utils.get_entities_by_gid handles the case of returning unknown entities in case there is a review for a release_group/event/place that was deleted (in future) from the musicbrainz database.
This needs to be fixed.