Re-add coupon / discount code functionality

[vdavid]

Context:
For the June release, we decided we'd momentarily let go of the "coupon code" functionality in the checkout process, meaning that when we roll out the Dotcom-based front end, users won't have a way to apply coupon codes to their purchases (except for making manual API calls, which is possible but not worthwhile as we don't have coupons for the general public anyway).

The back end is there, so it'd be easy to add the coupon code field, but then we'd need to display the discount and recalculate the total with a "preview" back-end call that we'd like to avoid for now. So instead, we decided to just remove this functionality.

Problem: We eventually want the coupon code functionality again to do promos, hand out to partners, etc.

Solution:
Display the coupon code option based on this figma, only if the showCouponCodeAtCheckout param to the checkout page is defined. Verify it on the back end and apply it to the transaction.

Note: @chrsmith's comment here in the figma might help, too.