feat(paywall): loadCheckoutModal now resolves when the modal is closed

[julien51]

Description

By resolving we let 3rd party applications handle things on their own when the modal is closed.
This is requested by the team at Coinage/Best Dish Ever.

Checklist:

• 1 PR, 1 purpose: my Pull Request applies to a single purpose
• I have commented my code, particularly in hard-to-understand areas
• I have updated the docs to reflect my changes if applicable
• I have added tests (and stories for frontend components) that prove my fix is effective or that my feature works
• I have performed a self-review of my own code
• If my code involves visual changes, I am adding applicable screenshots to this thread

Release Note Draft Snippet