Fix blank popup window bug on Firefox (#1610)

Resolves #1609

There's a bug that affects (at least) Firefox on Win10 where if the user
tries View > Dedicated Window, the window comes up blank and the JS
console has errors about "can't access dead object".

The issue seems like a bug in Firefox. What seems to be happening is
that the popup is trying to share resources with the main window, but
the browser's garbage seems to not be aware of the additional reference.
So the popup starts loading, we redirect the original window, which
causes the browser to tear down its resources, and then the popup
continues loading, but the shared resources it was trying to access have
been garbage collected in redirecting the original window.

Adding `noopener` seems to fix the issue, as it prevents the two windows
from sharing resources:

>`noopener`
>
>If this feature is set, the new window will not have access to the
originating window via Window.opener and returns null.
>
>When `noopener` is used, non-empty target names, other than _top,
_self, and _parent, are treated like _blank in terms of deciding whether
to open a new browsing context.

https://developer.mozilla.org/en-US/docs/Web/API/Window/open#parameters

Author: mtlynch

app/static/js/app.js

@@ -342,7 +342,11 @@ menuBar.addEventListener("dedicated-window-requested", () => {
   window.open(
     "/?viewMode=standalone",
     undefined,
-    `popup=true,width=${width},height=${height}`
+    // We need to add noopener to prevent a bug on Firefox where tearing down
+    // the existing page causes the browser to garbage collect resources that
+    // the popup tries to access.
+    // https://github.com/tiny-pilot/tinypilot/issues/1609
+    `popup=true,noopener,width=${width},height=${height}`
   );
 
   // Redirect the user to a placeholder page. We can’t keep the main window