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'd think something like:

```ruby
    where("tags @> :key => ARRAY[:value]", key: key, value: => value)
```

would be the safe way to express this (but haven't tested it).