Add rate_limit and message_limit to SerialisedService

The API needs these to check whether a service can send a notification.
This commit also updates all the tests in `test_validators.py` to take
a serialised service, not a database object.

app/serialised_models.py

@@ -120,7 +120,9 @@ class SerialisedService(SerialisedModel):
         'active',
         'contact_link',
         'email_from',
+        'message_limit',
         'permissions',
+        'rate_limit',
         'research_mode',
         'restricted',
     }