Mark grouphashes seen, then delete unseen ones

Once we have a sense of the rate at which we see distinct grouphashes and are confident we can safely do this, apply the new-years-hanger effect to grouphashes:

- Run a migration to mark them all unseen (new status value)
- Each time we see one, mark it seen
- Wait 90 days
- Run another migration to delete any which haven't been seen

Once metadata is fully rolled out (and so we can distinguish newly-created hashes from old ones by the `date_added` value, or lack thereof), we might start by just deleting all older grouphashes missing a group.