shopping cart coupon checkout changes as suggested by Jason Bau

@cdodge This PR contains the following change set : 

Moved the model "CourseRegistrationCode" from student to shoppingcart app.
Rename the model "Coupons" to "Coupon".
Added the "price_list" field in orderitem model. (in order to build the track of orignal price)
Updated the migration files.
Remove the logic related to discount price and updating the unit_cost of an item. 
Update the query in "get_total_amount_of_purchased_item" class method. 
Replace the variable purchased_cart_items to cart_items
Updated the list and receipt templates ( referencing to unit_cost rather then discount_price)
logging the coupon code during removal of coupon redemption.
updated the test cases of shopping cart.