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 only thing that matters for this is that all apps (ie both specialist publisher and finder frontend) generate the same identifier URL for the same topic.

Once we swap to using tags this becomes a non-issue anyway, so I'm not bothered if it's a reachable URL.