Compatibility with jQuery 1.9+ (use of $.fn.attr)

In several JS files in Webform we're using the $.fn.attr method to check the disabled/checked state of elements. Unfortunately, this usage of $.fn.attr is incorrect as of jQuery 1.9, and using it can lead to inconsistent results. To solve this, we should check if $.fn.prop exists and use it to check/set these properties. For more information, see http://api.jquery.com/prop.