Serialize user ids instead of users for delayed jobs

This will (hopefully) fix an issue we've had with active record objects being serialized incorrectly, causing emails to not be sent. This change keeps us from serializing active record objects buy serializing a ruby data structure (array of IDs) instead. There's an added benefit: when we deserialize, we make one batch query to get all users at once instead of N individual queries as each active record object is deserialized.