introduces the possibility to pull images from external registries
through Quay, storing them locally for faster subsequent pulls.
Closes PROJQUAY-3030 and PROJQUAY-3033
Since NamespaceGCWorker does a superset of RepositoryGCWorker's
operations, make sure that quay_gc_repos_purged is incremented if
either workers deletes a repository.
RepoMirrorConfig in the current database migration version has a
non-null constraint on the internal_robot field, but the model in
database.py does not.
Updates the model to match the current database revision, and handles
delete api calls when there are mirrors still using the robot.
Also set a default test DATABASE_SECRET_KEY when generating the test.db
