#2750701 composite-checkout: Add “free” payment method

Changes proposed in this Pull Request

This adds a “free” payment method for when the cart total is 0.

Testing instructions

• Run calypso with ENABLE_FEATURES=composite-checkout-wpcom npm start or use the ?flags=composite-checkout-wpcom query string when you get to checkout.
• Purchase a plan if you don’t have one already (I think this part is required?).
• Add only a domain mapping product to your cart. You can do this by visiting http://calypso.localhost:3000/domains/add/mapping/your-site-here
• Visit checkout and verify that the only payment method you see is “Woohoo! You don’t owe us anything!”.
• Complete checkout and be sure your purchase is successful.
• Verify that the Tracks events for the purchase look correct; notably calypso_checkout_composite_free_purchase_submit_clicked, and the payment_method property of calypso_checkout_composite_payment_complete.