Clean orphaned permissions after target delete

Cleans all orphaned Django-Guardian permissions after a Target is
deleted. This makes sense to do here as Targets can have row level
(private) permissions. We could potentially run this as well for
dataproducts, though I'm less versed on how it works there. Either way,
there's no reason to keep these objects around for any model, so it's
fine to delete permissions that aren't associated with Targets.

Alternatively, we don't do any cleanup and leave it to the TOM developer
to run the management command manually:
https://django-guardian.readthedocs.io/en/stable/api/management/

Author: Fingel

tom_targets/signals/handlers.py

@@ -1,8 +1,10 @@
 from django.dispatch import receiver
-from django.db.models.signals import post_save
+from django.db.models.signals import post_save, post_delete
+from guardian.utils import clean_orphan_obj_perms
 
 from tom_dataproducts.models import ReducedDatum
 from tom_targets.sharing import continuous_share_data
+from tom_targets.models import Target
 
 
 @receiver(post_save, sender=ReducedDatum)
@@ -11,3 +13,11 @@ def cb_reduceddatum_post_save(sender, instance, *args, **kwargs):
     # and if they exist, attempt to share the new data
     target = instance.target
     continuous_share_data(target, reduced_datums=[instance])
+
+
+@receiver(post_delete, sender=Target)
+def cb_target_post_delete(sender, instance, *args, **kwargs):
+    # When a Target is deleted, clean up orphaned permissions.
+    # Note that this removes ALL orphaned permissions, not just those
+    # associated with this target.
+    clean_orphan_obj_perms()