(WIP) Integrate SaferPay payment processing

@jbau @dianakhuang 

I had to quickly (a few hours) code up a PoC for integrating a different payment processor (SaferPay). This is needed for a customer launch very soon.

I just wanted to brush this by you quickly to see if you had any general issues with the approach. I haven't coded up sufficient error handling, testing, and there's one place I'm hardcoding in a domain name (localhost:8000). So this isn't a formal code review request, just a request for some initial feedback.

Thanks