Mark letters from insolvency service

So that DVLA knows to process them separately to avoid issues.

tests/app/conftest.py

@@ -179,7 +179,7 @@ def _sample_service_full_permissions(notify_db_session):
     service = create_service(
         service_name="sample service full permissions",
         service_permissions=set(SERVICE_PERMISSION_TYPES),
-        check_if_service_exists=True
+        check_if_service_exists=True,
     )
     create_inbound_number('12345', service_id=service.id)
     return service