Related filter

Adds a new ?related=obj_id filter on artists, albums and tracks endpoints, as per https://github.com/apognu/otter/issues/24#issuecomment-643763282.

This is based on tags, and meant to be used like this to get the most relevant entries:

GET /api/v1/artists?related=42&ordering=-related

changed milestone to %0.21.2

changed the description

changed target branch from develop to master

enabled an automatic merge when the pipeline for d50cce36 succeeds

mentioned in commit c8f9ac97

merged