code: function requireExtensionJs(sessionId) {
      var src = 'scripts/user-extensions.js[' + sessionId + ']';
      if (document.getElementById(src) == null) {
          var scriptTag = document.createElement('script');
          scriptTag.language = 'JavaScript';
          scriptTag.type = 'text/javascript';
          scriptTag.src = src;
          scriptTag.id = src;
          scriptTag.charset = 'UTF-8';
          var headTag = document.getElementsByTagName('head')[0];
          headTag.appendChild(scriptTag);
      }
  }
docs: Adds a script tag referencing a specially-named user extensions "file". The
  resource handler for this special file (which won't actually exist) will use
  the session ID embedded in its name to retrieve per-session specified user
  extension javascript.
  
  @param sessionId