Notify sponsors about ending roles

The setup has five parts:

• a new email template type
• a new django-q task for notifying
• a management command for scheduling the task periodically
• a new settings variable NOTIFIER_LIMIT for controlling the number of days into the future the notifier script should use for end date of roles.
• a new settings variable NOTIFIER_SCHEDULE_TYPE for controlling how often the notifier task should be scheduled when using the management command.

Resolves: GREG-162

greg/management/commands/import_sponsors_from_cerebrum.py

@@ -40,6 +40,7 @@ class Command(BaseCommand):
             organizational_unit=unit,
             source=self.CEREBRUM_SOURCE,
             automatic=True,
+            hierarchical_access=False,
         )
         if created:
             logger.info("sponsor_ou_link_create", sponsor=sponsor.id, sunit=sunit.id)
@@ -119,7 +120,7 @@ class Command(BaseCommand):
         active_units = OrganizationalUnit.objects.filter(
             active=True,
             deleted=False,
-        )
+        ).exclude(identifiers__value__in=[])
 
         logger.info("import_start", nr_of_units=len(active_units))
         for unit in active_units: