Implement a generalized order receipt email system

[rhysyngsun]

We want to implement a receipt system so that we can use it in MITx Online and in the future update our existing implementations to utilize this as well.

In the near term we need it for this: mitodl/mitxonline#532

Inspiration

We have some existing implementations for receipt emails, the most comprehensive of which is on xPro:

https://github.com/mitodl/mitxpro/blob/master/ecommerce/mail_api.py

Note that xPro has a couple of different types of emails

Tasks

• Add a mitol.receipts application
• Add a base receipt email message class based on TemplatedMessage
• Add some email templates for our standard receipt email design in the receipts app. This should ideally be broken down into several subtemplates to make it easy to override parts of the email if necessary without having to rewrite the whole thing.