Add rel="noopener noreferrer" to external links

Background:
https://medium.com/@jitbit/target-blank-the-most-underestimated-vulnerab
ility-ever-96e328301f4c#.hduwdbjlb

core/modules/utils/dom/modal.js

@@ -102,6 +102,7 @@ Modal.prototype.display = function(title,options) {
 		var link = document.createElement("a");
 		link.setAttribute("href",tiddler.fields.help);
 		link.setAttribute("target","_blank");
+		link.setAttribute("rel","noopener noreferrer");
 		link.appendChild(document.createTextNode("Help"));
 		modalFooterHelp.appendChild(link);
 		modalFooterHelp.style.float = "left";