Add purchase link to access plan

[brianhogg]

Description

Also ensures that the URL is valid if the checkout URL doesn't contain the domain.

Fixes #2793

How has this been tested?

Manually

Screenshots

Checklist:

• [ ] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [ ] My code has been tested.
• [ ] My code passes all existing automated tests.
• [ ] My code follows the LifterLMS Coding & Documentation Standards.