FEATURE: Use new Badging API

Now that the spec is finished use the unprefixed API, which was also moved
from window to navigator.

Still uses feature detection so it fail gracefully when not available in
the user agent.

app/assets/javascripts/discourse/initializers/badging.js.es6

@@ -4,7 +4,7 @@ export default {
   after: "message-bus",
 
   initialize(container) {
-    if (!window.ExperimentalBadge) return; // must have the Badging API
+    if (!navigator.setAppBadge) return; // must have the Badging API
 
     const user = container.lookup("current-user:main");
     if (!user) return; // must be logged in
@@ -18,6 +18,6 @@ export default {
   },
 
   _updateBadge() {
-    window.ExperimentalBadge.set(this.notifications);
+    navigator.setAppBadge(this.notifications);
   }
 };