Gifting: Checkout cancellation should return to referrer not /start/domain.

[lsl]

When clicking the "X" in checkout or otherwise emptying the cart you should be returned to the referring domain.

Some quick testing of https://wordpress.com/checkout/business-bundle/gift/19766713?cancel_to=https://... and checking the code tells me we're only able to redirect to internal routes, lets just go to home for now.

Acceptance:

• [ ] Add a ?cancel_to=/ url param to the Gift CTA url so early checkout exit's return somewhere other than /start/domain

[lupus2k]

What do you think about adding an else here using window.location.href?

[lupus2k]

Created this diff (for simple sites): D93366-code Also, this PR at wpcomsh (for atomic sites): https://github.com/Automattic/wpcomsh/pull/1157