Include the order number in customer mails

Previously, the order number was not included in the mails that are sent out to the customer, which was making it difficult for customers to reference their orders. The order number has been added to the subject line of both mails sent from the application.

https://trello.com/c/epeMCNef

![](http://www.reactiongifs.com/r/6j6t9iT.gif)