Add a shipment notification flash

Previously, there was no way for an administrator to know that the shipment notification email had been sent successfully, which was causing the email to be sent out multiple times unneccessarily. A flash has been added to be shown when the administrator successfully sends the shipment notification email.

https://trello.com/c/ex8LQ8dA

![](http://www.reactiongifs.com/r/tys.gif)