Add Public Post Preview compatibility shim to RSA

[benlk]

Description of the Change

This substantially integrates the shim wp-plugin-ppp-rsa-compat.php into Restricted Site Access.

The shim reimplements significant portions of Public Post Preview's functionality, because the methods in PPP used to verify whether the preview link is valid are set to private access.

I'm filing this PR against RSA, but if this feature is better implemented in PPP, I have a corresponding PR against PPP here: ocean90/public-post-preview#184

Resolves #344

How to test the Change

1. Enable PPP and RSA on the same site.
2. Create a draft post.
3. Verify that the draft post is visible to logged-in users, and blocked for logged-out users.
4. Enable PPP for the post.
5. Verify that the draft post is visible to both logged-in users and logged-out users.
6. Disable PPP for the post.
7. Verify that the draft post is visible to logged-in users, and blocked for logged-out users.

Changelog Entry

Added - Compatibility with Public Post Preview to allow logged-out users to view preview links.

Credits

Props @benlk

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.

[benlk]

This PR is a draft because, while the shim has been tested on live sites, I have not tested this specific PR's code in any environment. There's a PHPCS fail, and tests and docs are needed before this is ready for merge.

However, this PR should be sufficient to determine whether this PR or ocean90/public-post-preview#184 is preferred.