Add payment information to payment confirmation page

* Add JSON end-point for shoppingcart receipt information
* Add enrollment confirmation page
* Fix progress update
* Add order info to the payment confirmation page.

I tried as much as possible to isolate `verify_student` from `shoppingcart`.  The step responsible for payment information uses a querystring param to find the order number; this will need to be set from the post pay callback redirect.  This step then queries the shopping cart to retrieve a JSON representation of the receipt, which it uses to render the page.

@rlucioni This also fixes the issue with the progress steps not being updated that you noticed yesterday.

Reviewers: @stephensanchez @rlucioni 
FYI: @dianakhuang @chrisndodge 
This works, but I could imagine a cleanup creating a DRF view and serializers for Orders / OrderItems that would avoid all the manual serialization.