[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.

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<div id="ajaxroot">

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  <div class="ajaxHtmlHead">

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    <script src="http://foo.js" type="text/javascript"> </script>

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    <link rel="stylesheet" type="text/css" media="all" href="qunit.css" />

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  </div>

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  <h1>Hello</h1>

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</div>