Reduce notification amount for roles

If other fields than one of the date fields was changed, an extra task
would be created, causing duplicate notification on the date. We now
check that the field has changed before making a task.

greg/signals.py

@@ -129,19 +129,23 @@ def save_notification_callback(sender, instance, created, *args, **kwargs):
         return
     # Queue future notifications on start and end date for roles
     if isinstance(instance, Role):
-        if instance.start_date:
+        if (
+            "start_date" in instance._changed_fields  # pylint: disable=protected-access
+            and instance.start_date
+        ):
             Schedule.objects.create(
                 func="greg.signals._queue_role_start_notification",
                 args=f"{instance.id},True",
                 next_run=date_to_datetime_midnight(instance.start_date),
                 schedule_type=Schedule.ONCE,
             )
-        Schedule.objects.create(
-            func="greg.signals._queue_role_end_notification",
-            args=f"{instance.id},True",
-            next_run=date_to_datetime_midnight(instance.end_date),
-            schedule_type=Schedule.ONCE,
-        )
+        if "end_date" in instance._changed_fields:  # pylint: disable=protected-access
+            Schedule.objects.create(
+                func="greg.signals._queue_role_end_notification",
+                args=f"{instance.id},True",
+                next_run=date_to_datetime_midnight(instance.end_date),
+                schedule_type=Schedule.ONCE,
+            )
     meta = _create_metadata(instance)
     operation = "add" if created else "update"
     _store_notification(