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.  
Agreed, this is a very good point. Do we have a thought on where best we should handle this. Do we want to maintain a second set of observers for this. My worry would be that the 'republication_observers' and the 'publication_observers' would get out of sync although this could be remedied simply with a base_publication observers. Perhaps a better option would be to handle this in EmailAlertExporter class although this does not feel like the responsibility of that class.