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
If the record has multiple entries in the "topics" tag, this will drop all but the one being renamed.

eg, topics => ["oil-and-gas/licensing", "oil-and-gas/taxation"], which would represent a subscription to emails for updates from documents that have either of those tags.

We don't currently have such subscriptions, but they're allowed by the data model, so at the least we should produce a noisy error if we see one.