fix(content-gate): persist restricted content

[miguelpeixe]

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:

Filters applied to the_content may restore the original content to a restricted post. This ensures the restricted content persists.

How to test the changes in this Pull Request:

1. Without Woo Memberships, configure a content gate (Audience -> Content Gates) with inline layout
2. While in trunk, visit a restricted post and confirm the gate never renders, and the original post is displayed
3. Checkout this branch, run wp cache flush, and refresh the page
4. Confirm the content is restricted, and the gate now renders in the expected position
5. Change the gate layout to overlay and confirm it renders as expected and the content is also restricted

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?

[dkoo]

This isn't quite working for me. Caching the content does seem to work well, but it also causes inline gate layout post content to get rendered without content filtering, meaning the raw block markup is rendered instead of the transformed HTML.

Separately, not due to anything in this PR, if both Registration and Paid Access layouts are enabled, visiting a restricted post will cause the Paid Access layout to always appear and never the Registration layout. This might be intentional, but I'm not sure it should be the case.

#4421 proposes some fixes which may address both of these issues, but let me know if it warrants further discussion.