Block edit interface resize gets mystery right margin - Fix #412

It a jQuery UI problem when calculating widht/height on elements with 'box-sizing: border-box;'.
You can even reproduce this on Themeroller (using box-sizing) on the modal window:
 - http://jqueryui.com/themeroller/

But not on their demo page: (no box-sizing )
 - http://jqueryui.com/dialog/#modal-form

jQuery UI Bug Tracker: 
 - http://bugs.jqueryui.com/ticket/9832
 - http://bugs.jqueryui.com/ticket/8506

I thought it may be better to use this simple css fix instead of having a jQuery UI patch (https://github.com/jquery/jquery-ui/pull/713/files).