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.

* 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

[Trello ticket](https://trello.com/c/jxx7DlUA/330-endpoint-to-send-emails-given-a-set-a-of-tags-5)
I know you're already using this all over the place, but we're using [`format`](https://github.com/alphagov/content-store/blob/master/doc/document_format.md#format) to describe what I think this field is everywhere else, so we should try to be consistent.