Bring in the responsive component of MP6. See #25858.

* Makes the admin fully responsive down to 320px wide.
* Adds a touch-optimized main menu that can be opened and closed from the toolbar.
* Size and positioning adjustments to icons, buttons, and text elements for better touch usability.

A few changes since MP6:

* Removed jQuery mobile. This script was used to add swipe controls to open/close the sidebar menu. This feature was apparently buggy and due to the pending demise of jQuery mobile, it was removed.
* Removed use of Backbone.js. Adding Backbone.js to this script would add a dependency of Backbone.js for all of the admin. Additionally, it was used to add a menu item. Instead of doing that, it was added via the admin menu API. This also fixes a bad delay in the item showing in the menu.
* CSS layout is standardized. Comments have also been cleaned up.
* Jetpack and Akismet code is removed.
* RTL CSS is removed.
* JS passes hinting other than one small issue that will likely be removed when parts of the code are reviewed.

A number of areas for improvement remain; we're tracking these issues in the comments of #25858.

Props to tollmanz, tillkruess, helen, dd32, and apeatling.



git-svn-id: https://develop.svn.wordpress.org/trunk@26134 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/admin-header.php

@@ -67,6 +67,7 @@ var ajaxurl = '<?php echo admin_url( 'admin-ajax.php', 'relative' ); ?>',
 	decimalPoint = '<?php echo addslashes( $wp_locale->number_format['decimal_point'] ); ?>',
 	isRtl = <?php echo (int) is_rtl(); ?>;
 </script>
+<meta name="viewport" content="width=device-width,initial-scale=1.0">
 <?php
 
 /**