Only set deleted date once

[peer35]

Update filter here, or the system keeps updating the deleted date. Also note #17

def cleanup():
    days = 2
    last_update = MiscStats.objects.order_by('collected').last().collected
    cutoff = make_aware(datetime.combine(last_update, datetime.min.time())) - timedelta(days=days)
    logger.info(f'Mark folders and datasets last updated before {cutoff} as deleted.')
    ResearchFolder.objects.filter(updated__lte=cutoff).update(deleted=now())
    VaultFolder.objects.filter(updated__lte=cutoff).update(deleted=now())
    VaultDataset.objects.filter(updated__lte=cutoff).update(deleted=now())