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
Having log information about what this call does would be very helpful.  Ideally, a line printed for each change made.

This should probably be done by making a logger and passing it to the TagChanger.