Audit permissions when adding a user to a service

This is useful information to store for the event, which would be
lost if someone subsequently changed them.

Rather than updating lots of mock assertions, I've replaced them
with a single test / assert at a lower level, which is consistent
with auditing being a non-critical function.

tests/app/test_event_handlers.py

@@ -49,7 +49,8 @@ def test_create_add_user_to_service_event_calls_events_api(client, mock_events):
     kwargs = {
         "user_id": str(uuid.uuid4()),
         "invited_by_id": str(uuid.uuid4()),
-        "service_id": str(uuid.uuid4())
+        "service_id": str(uuid.uuid4()),
+        "admin_roles": {'manage_templates'},
     }
 
     create_add_user_to_service_event(**kwargs)