Send alerts

Send an alert (notification) containing subject and body to a topic (soon to be renamed subscriber list) based on tags from the publication.

[Trello ticket](https://trello.com/c/jxx7DlUA/330-endpoint-to-send-emails-given-a-set-a-of-tags-5)

* GovDelivery API client simplified, just returns a Struct with whatever data is in the response XML
* Generate proper UUIDs for us to identify our subscriber lists