Fixes #276: Delete RoleImages when their DeviceRole is deleted (#279)

* [fix] get only valid role images

* [up] add post delete signals for device roles

* [fix] move signal registration in `apps.py`

* [fix] register signals in plugin config & filter valid role images

* [fix] undo role image filter as it is wrong and unnecessary

* [up] revert formatting with black

netbox_topology_views/signals.py

@@ -0,0 +1,14 @@
+from typing import Type
+
+from dcim.models import DeviceRole
+from django.contrib.contenttypes.models import ContentType
+from django.db.models.signals import pre_delete
+from django.dispatch import receiver
+
+from netbox_topology_views.models import RoleImage
+
+
+@receiver(pre_delete, sender=DeviceRole, dispatch_uid="delete_hanging_role_image")
+def delete_hanging_role_image(sender: Type[DeviceRole], instance: DeviceRole, **kwargs):
+    ct = ContentType.objects.get_for_model(sender)
+    RoleImage.objects.filter(content_type=ct, object_id=instance.id).delete()