Protect against phishing attacks on target _blank links.

app/com/gitpitch/views/frags/SlideshowReveal.scala.html

@@ -185,6 +185,7 @@
         function upgradeAnchors() {
             Array.from(document.getElementsByTagName('a')).forEach($link => {
               if ($link.hostname !== window.location.hostname) {
+                $link.setAttribute('rel', 'noopener noreferrer');
                 $link.setAttribute('target', '_blank');
               }
             });