WIP: Added stripe integration

[karbassi]

Stripe integration works as expected. We decided to remove the dj-stripe module and go with our own implementation.

Tasks

• Create a new "app" in django that will handle all payments.
• Create a single donation based on donation amount buttons or what the user enters
  • When creating a donation, the donor should show up as a "customer" in the stripe backend
  • When a donation is created, it should be recorded in the django backend.
  • If the user is logged in, we need to associate their donation to their account
  • If the user is not logged in, just recorder their first name, last name, and email in the dj-stripe system
• Create a single "buy" setup for a single class.

Related to #588