KSES: Allow the `download` attribute on `<a>` tags.

To avoid this being a vector for bypassing the filetypes that are allowed to be uploaded, this attribute is only allowed to be added without a value.

Props kalpshit, arshidkv12, welcher, peterwilsoncc, marina_wp, pento.
See #44724.


Built from https://develop.svn.wordpress.org/branches/5.0@43813


git-svn-id: http://core.svn.wordpress.org/branches/5.0@43642 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/kses.php

@@ -66,6 +66,9 @@ if ( ! CUSTOM_TAGS ) {
 			'rev' => true,
 			'name' => true,
 			'target' => true,
+			'download' => array(
+				'valueless' => 'y',
+			),
 		),
 		'abbr' => array(),
 		'acronym' => array(),

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-43812';
+$wp_version = '5.0-alpha-43813';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.