KSES: Add `flex` and related long form properties to safe CSS.

Allow `flex`, `flex-grow`, `flex-shrink` and `flex-basis` to be used in inline CSS. As of WordPress 5.3 the block editor is expected to use `flex-basis` inline to set the width in the column block.

Props aduth.
Fixes #47281.
See #37248.


Built from https://develop.svn.wordpress.org/trunk@45363


git-svn-id: http://core.svn.wordpress.org/trunk@45174 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/kses.php

@@ -2063,6 +2063,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 	 * @since 5.0.0 Added support for `background-image`.
 	 * @since 5.1.0 Added support for `text-transform`.
 	 * @since 5.2.0 Added support for `background-position` and `grid-template-columns`
+	 * @since 5.3.0 Added support for `flex`, `flex-grow`, `flex-shrink`, and `flex-basis`.
 	 *
 	 * @param string[] $attr Array of allowed CSS attributes.
 	 */
@@ -2133,6 +2134,11 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'padding-left',
 			'padding-top',
 
+			'flex',
+			'flex-grow',
+			'flex-shrink',
+			'flex-basis',
+
 			'clear',
 			'cursor',
 			'direction',

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45362';
+$wp_version = '5.3-alpha-45363';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.