Coupon discount is wrong in checkout for multi-year domains

[sirbrillig]

The coupon discount amount shown by checkout does not include product volume and therefore is incorrect for multi-year domains. In addition, since this number is currently being used to calculate the "Subtotal" being displayed in checkout, that number is also wrong.

This has been broken for a very long time. Probably ever since we started showing a coupon discount amount. We never noticed because very few products have used volume until multi-year domains.

To reproduce:

• Add a domain to your cart that is not on sale.
• Add a coupon to your cart.
• Verify the math of the coupon savings and the subtotal are wrong.

[sirbrillig]

Will be fixed by D134952-code. I'll see if I can fix it sooner.

Here's how it should look: