Add `<s>` to `$allowedtags` in KSES.

From https://developer.mozilla.org/en-US/docs/Web/HTML/Element/s :
"The <strike> element, alter ego of the <s> element is obsolete and should not be used on Web sites any more."

`<strike>` shall remain for BC, but `<s>` should not be stripped from the author (et al) role's HTML input.

Props paulschreiber.
Fixes #30954.

Built from https://develop.svn.wordpress.org/trunk@31205


git-svn-id: http://core.svn.wordpress.org/trunk@31186 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/kses.php

@@ -441,6 +441,7 @@ if ( ! CUSTOM_TAGS ) {
 		'q' => array(
 			'cite' => true,
 		),
+		's' => array(),
 		'strike' => array(),
 		'strong' => array(),
 	);

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-alpha-31204';
+$wp_version = '4.2-alpha-31205';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.