Fix unsafe link

https://web.dev/external-anchors-use-rel-noopener/?utm_source=lighthouse&utm_medium=devtools

src/js/pannellum.js

@@ -160,7 +160,7 @@ uiContainer.appendChild(dragFix);
 // Display about information on right click
 var aboutMsg = document.createElement('span');
 aboutMsg.className = 'pnlm-about-msg';
-aboutMsg.innerHTML = '<a href="https://pannellum.org/" target="_blank">Pannellum</a>';
+aboutMsg.innerHTML = '<a href="https://pannellum.org/" target="_blank" rel="noopener noreferrer nofollow">Pannellum</a>';
 uiContainer.appendChild(aboutMsg);
 dragFix.addEventListener('contextmenu', aboutMessage);