hgweb: add docFromHTML javascript function

It takes a string with HTML markup and creates DOM document from it.
The implementation is a part of code from
https://developer.mozilla.org/en-US/docs/Web/API/DOMParser#DOMParser_HTML_extension_for_other_browsers

mercurial/templates/static/mercurial.js

@@ -328,3 +328,9 function makeRequest(url, method, onstar
     onstart();
     return xfr;
 }
+
+function docFromHTML(html) {
+    var doc = document.implementation.createHTMLDocument('');
+    doc.documentElement.innerHTML = html;
+    return doc;
+}