Add Site Editor and PHP changes from Gutenberg 10.1 - 11.9

- First pass at adding the site editor from the Gutenberg plugin to
  wp-admin/site-editor.php.
- Adds miscellaneous PHP changes from Gutenberg 10.1 - 11.9.

Follows [52042].
See #54337.
Props youknowriad, aristath, hellofromtonya, gziolo.


git-svn-id: https://develop.svn.wordpress.org/trunk@52069 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/rest-api/rest-users-controller.php

@@ -207,6 +207,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 				'search',
 				'slug',
 				'who',
+				'has_published_posts',
 			),
 			$keys
 		);