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
Remove “Pay type” field

Previously, there was a “Pay type” field on the new order page, which
is no longer required because all payments are made using a credit or
debit card. The field has been removed from the new order form.

https://trello.com/c/dinpdxpz