fix(modal-checkout): empty cart when modal checkout is abandoned

[chickenn00dle]

All Submissions:

• [ ] Have you followed the Newspack Contributing guideline?
• [ ] Does your code follow the WordPress' coding standards and VIP Go coding standards?
• [ ] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1207817176293825/1208620127411764/f

This PR adds an ajax request to empty cart when modal checkout is abandoned.

How to test the changes in this Pull Request:

1. Via any checkout or donate button open modal checkout as a reader
2. In another tab (in the same session) open the cart page test.site/cart
3. Verify the item is present in the cart
4. Back in the modal checkout tab, close the checkout modal without completing checkout
5. Go back to the cart tab. On epic/ras-acc the item will still be present. On this branch, the cart should be empty.

Other information:

• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?
• [ ] Have you successfully ran tests with your changes locally?

[chickenn00dle]

Thanks for the review @miguelpeixe!

Once I authenticate the nonce provided on page load will no longer be valid.

Ah whoops! I always forget about the transition in login state 🤦 . I moved the nonce to the checkout script and pulled in modal.js as you suggested: https://github.com/Automattic/newspack-blocks/pull/1923/commits/7a1c99ac92428484667f5e187da7a3f53ab58f95