Email notification when review complete

[StuMaitland]

Really enjoying using this as part of a moderation workflow. At the moment although reviewers are notified by email when a review has been sent, the original user isn't notified when the review is completed.
I can code this if you're tight on time at the moment, but just to suggest what my approach would be:

• New method models.Share.send_response_email()
• Call send_response_email() in api.views.Respond.post()

Along with a new template for the response email.