#2639802 composite-checkout: Hook up coupon field

I recommend looking at the commits in order. Depends on D38431 and D38473.

Changes proposed in this Pull Request

• This PR updates composite checkout in wpcom to pass coupons to the cart endpoint and display a notice when the coupon is added successfully.

To do:

• View handling for ‘pending’ coupon submission (disable field and button, set button label to “Processing…”)
• Hide the coupon field if one has already been applied
• Add a coupon line item to the order review and order summary steps
• Make the coupon field in the summary active
• Show an error message on the field if the coupon is invalid
• Hide calypso notification on failure
• Bug fix: “Processing…” button does not appear on the review coupon field if the summary field is visible
• Bug fix: show original line item costs in review step
• Reset field error state on input
• Bug fix: Backspace twice on review order coupon field triggers step number change event
• Bug fix: Typing into review order coupon field is wonky – autocomplete appears in the wrong place and textbox loses focus on each input event
• Bug fix: Every input event in summary coupon field retriggers ‘Apply’ button animation in review order field
• Update: The deployed version of D38431 uses coupon_discounts_integer; need to update the response cart type to reflect this.

Testing instructions

npm run test-packages composite-checkout-wpcom

Manual steps to test basic behavior:

1. Do not sandbox the store.
2. Enter checkout with some products and proceed to the ‘review order’ step.
3. Try to apply an invalid coupon; that is, a string not consisting only of alphanumeric characters, ‘-‘, and ‘_’. Verify that the coupon field is unhappy.
4. Try to apply a valid but not active coupon such as ‘blahtestcoupon’. Verify that you see a field error and that your item and total prices do not change.
5. Try to apply an active coupon. (Check with me or the field guide about how to find these.) Verify that you see a notice and that your item and total prices change appropriately, and that a ‘coupon’ line item appears in the cart with the correct price.
6. Enter checkout again with a fresh cart and repeat steps 3-5, this time using the “Add coupon” link in the order summary step.

Edge cases:

• Simulate entering the cart with a coupon already applied by refreshing the page after adding a coupon. Check that the coupon line item appears in the summary step, and that the “Add a coupon” button is not visible.