[RichString] Add markdown support Supporting markdown requires the 'Markdown' python packages, and since it widely available on the target platforms adding it as a strong requirements will not be a big constraint. Closes #3814302

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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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  </div>

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  <h1>Hello</h1>

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</div>