jquery: Add the js class, remove the no-js one

This used to be done by Modernizr until its removal in
08f27c1. Some sites apply different styling for
JS-disabled environments; one such example is the jQuery UI Download Builder
which expands all Themeroller knobs sections in the no-JS mode.

Closes gh-457

themes/jquery/js/main.js

@@ -1,3 +1,6 @@
+// Some CSS depends on the `js` class.
+$( "html" ).addClass( "js" ).removeClass( "no-js" );
+
 /*
  * All sites
  */