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
Correction, it would be something like:

```ruby
    where("tags @> :key => :value", key: key, value: [value].to_json)
```