Consider whether events.labels and subscription.labels should be lists of references

[osteele]

http://docs.mongoengine.org/guide/defining-documents.html#one-to-many-with-listfields documents and demonstrates the use of a MongoDB DBRef (via MongoEngine's ReferenceField) to preserve database integrity.

This would remove the need to update all the event.labels and subscription.labels when a label is renamed.

It would also allow us to define the behavior when a label is deleted.