Stanford paid course registration

With tests, some settings changes
(all should default to not breaking anything for edx)

Added styling for shopping cart User Experience
- Styled shoppingcart list page
- Styled navigation shopping cart button
- Styled receipt page
- Styled course about page for shopping cart courses

@dianakhuang and @talbs for the styling/small bit of front-end.

rake coverage at 100%, rake quality at 100%