fix: add thankyou hook to custom form-login template

[dkoo]

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:

#1550 implemented a custom form-login template that shows the regular "thank you" template order summary even when completing a transaction with an already-registered email address. #1521 added an optional redirect button to the regular thank you template. This PR adds the custom hook required to show the redirect button on the custom form-login template.

How to test the changes in this Pull Request:

1. Follow instructions in feat: add option for a button after checkout #1521 to add a redirect button to a Checkout Button block.
2. Complete a transaction using the Checkout Button, using an email address that's already registered with your test site.
3. On master, observe that the redirect button is not shown.
4. On this PR, confirm it is shown.

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?

[adekbadek]

Note: these changes will be made obsolete if #1561 is merged.

[adekbadek]

Closing, since #1561 fixes it among other changes.