Rake task to update subscriber list tags

When a topic changes, we should update the Subscribers List in Email
Alert API to keep subscribers updated.
This should be done by creating a new entry for the updated topic with
the same govdelivery identifier.

https://www.pivotaltracker.com/story/show/83840958
I'm not sure this makes sense as a member of SubscriberList.  At the least, its name needs to be more descriptive: "format_value" is generic enough that it doesn't mean much.