Send publication alert with tags

This pull request allows for sending of emails on publication of a document. This includes 2 parts.

The first part of this is to remove the dependency on govuk_delivery and introduce the use of the email-alert-api. We have switched over to use the new API in order to be able to send tags to the api so that email-alert-api can decide which "topics"/"subscriber_lists" to notify of a publication. 

The second part is to introduce the use of tags in the notification in order to decide the correct subscriber lists which need to be notified. This differs from the previous implementation as the "topics" in govuk_delivery previously only corresponded to an individual finder's document formats, not filtered sets of document formats such as only drug alerts or only device alerts.

Trello story here:
https://trello.com/c/z5I45pxb/402-email-an-email-is-sent-to-subscribers-when-a-document-within-a-finder-is-published