Add a verification reminder to order confirmation emails

@wedaly here's my take on the verification reminder. Is this what you had in mind?
I'm unsure of how to get the protocol. I can't take advantage of the `is_secure` property on requests here. And the `purchase` function, which calls `send_confirmation_emails`, [doesn't bother with the protocol at all](https://github.com/edx/edx-platform/blob/master/lms/djangoapps/shoppingcart/models.py#L434). `send_confirmation_emails` ends up constructing a dashboard URL whose [base is just the domain](https://github.com/edx/edx-platform/blob/master/lms/djangoapps/shoppingcart/models.py#L326).