Update delete_expired_artifacts to take collections into account

https://freexian-team.pages.debian.net/debusine/explanation/concepts.html documents the intended rules for the retention of artifacts referenced by collections, and for the retention of collection items. We should update delete_expired_artifacts for this.

https://freexian-team.pages.debian.net/debusine/reference/collections.html#collection-workflow-internal also requires that a workflow's internal_collection should not retain its contained artifacts. I suggest adding a retains_artifacts flag to the Collection model for this (or a better name if you can think of one).

I think it would make sense to implement expiry of collection items in the delete_expired_artifacts command. It should happen before running expiry on artifacts themselves.

• add Collection.retains_artifacts
• set retains_artifacts on workflow internal_collections (may be moved to another issue if the relevant bits of workflows aren't ready yet)
• don't delete artifacts as long as they are referenced by a collection with retains_artifacts=True; this should be done somewhere within DeleteExpiredArtifacts._mark_to_keep
• implement expiry of collection items