More cleanly transition to mandatory :payment_integration parameter in URL

[gregorbg]

Basically a copy-cat of #10458, except that the parameter in the URL is not marked as required yet.

This will make it so that if anybody starts a Stripe session with the old (paremeter-less, because optional) URL during deployment, the route would still resolve after coming back to the newly deployed server.

Highly unlikely circumstance anyways (somebody would need to perform their 3DS authorization exactly during the time window that AWS swaps out our ECS instances and switches over the traffic) but with payments you can never be safe enough.

Since the original #10458 is already reviewed and approved, I will merge this as soon as tests pass.

[gregorbg]

Merging based on the review of #10458

This PR literally cherry-picks the exact same commits (and squashes them, so the hashes won't match. But the diff does!)