Send email alerts on publish

This is the work to allow emails to be sent on publish to the govuk_delivery API. Currently the formatting of the email is relatively unimportant to us, however it should be simple enough to change the view templates later. It would be nice to be able to render view using Erb directly in order to remove the exporter class' dependency on ActionView. This is something have looked into however it is not as simple as i first assumed. For now lets use ActionView in order for us to be able to test email sending.  
The republishing script calls all publication observers again when republishing a document (see https://github.com/alphagov/specialist-publisher/blob/master/lib/document_repository_observer_mapper.rb)

We absolutely need to make sure that in this pull request we change this behaviour or add on the observer elsewhere so that we don't renotify everyone about every single specialist document each time we republish the documents -- only when each edition is first published.