Add store credit display to checkout flow

- Shows store credit in side/summary bar
- Shows store credit when needing to enter in payment details, while
making it clear that the customer will not be charged
- Shows store credit in the order confirmation page as well as the order
show page
- A customer's order index page still shows the usual order total
(unchanged behavior)
Only show store credit on summary if it exists

Adds in using_store_credit? helper on order_decorator to assist in views
determining whether or not to show store credit