Mark grouphashes seen, then delete unseen ones

[lobsterkatie]

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.