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

src/wp-includes/kses.php

@@ -2287,6 +2287,8 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'border-right-width',
 			'border-bottom',
 			'border-bottom-color',
+			'border-bottom-left-radius',
+			'border-bottom-right-radius',
 			'border-bottom-style',
 			'border-bottom-width',
 			'border-bottom-right-radius',
@@ -2297,6 +2299,8 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'border-left-width',
 			'border-top',
 			'border-top-color',
+			'border-top-left-radius',
+			'border-top-right-radius',
 			'border-top-style',
 			'border-top-width',
 			'border-top-left-radius',