Override Site Editor URLs to use plugin page

[Mamaduka]

Description

PR add filter to override wp-admin/site-editor.php paths with themes.php?page=gutenberg-edit-site. It should prevent plugin users from accidentally accessing editor shipped with the core in 5.9.

Testing Instructions

1. Go to Themes and activate any block theme.
2. Click on the "Customize" button.
3. It should take you to the themes.php?page=gutenberg-edit-site page.

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[Mamaduka]

Thank you, Ari.

[noisysocks]

Why can't we use site-editor.php? I think that the Gutenberg plugin and Core should converge onto a single URL. For example, we don't use different URLs depending on whether the plugin is active for the widgets block editor or post block editor.

[Mamaduka]

The site-editor.php doesn't exist in WP 5.8, and the plugin still has to support that version.

We'll remove a separate page when the minimum required version is 5.9.