Updating checkout nonce via AJAX

[dparker1005]

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Changes proposed in this Pull Request:

When a user is created at checkout but the checkout fails (ex Stripe SCA) and the form is shown again, the nonce on that re-generated checkout form will not be specific to the newly created user object. This will cause nonce verification to fail when the form is then submitted again.

This PR adds JS to update the nonce value when the checkout form is displayed to ensure that we have the correct nonce value for the current user.

How to test the changes in this Pull Request:

1. Set up site with Stripe
2. Check out as new user using Stripe SCA test card
3. Choose to authenticate successfully
4. See that nonce fails before PR, but succeeds afterwards

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.