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.


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