[web] implement cross origin resource sharing (CORS) (closes #2491768) Partial implementation that is enough to get started but leaves out some of the advanced features like caching and non-simple methods and headers.

<div id="ajaxroot">
  <div class="ajaxHtmlHead">
    <script src="http://foo.js" type="text/javascript"> </script>
  </div>
  <h1>Hello</h1>
</div>