Add store credit display to checkout flow

Starts adding some UI for customers to see and deal with store credits. It is untested for the moment but maybe it can become the basis for some more frontend work if anyone is willing to pursue further!

Parts of the UI it attempts to cover:
- 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)