Send an alert when Inbox Placement has finished processing.

It'a quite troublesome to re-construct an html email in the Inbox Placement configuration, especially when our web framework may be creating it.

Since we've already been sending several types of transactional emails through mailgun, it would neat to be able to select one or more of them as tests since they're already being sent. (and it would be good to see if our existing emails are being placed efficiently!)