set rel attribute on external links to noreferrer

js/externalLinkHandler.js

@@ -3,6 +3,7 @@ document.onreadystatechange = function () {
          let href = element.href;
          if(href.indexOf(location.hostname) === -1){
              element.target = "_blank";
+             element.setAttribute("rel", "noreferrer");
          }
      });
-}
+}