Add an app event for when a topic is fully loaded

If we are interested in page events (say analytics), they are reported
when the route changes, which does not wait for any promises in
`setupController` to finish.

A plugin might want to know when a topic has fully loaded, so this event
is triggered when that happens.

app/assets/javascripts/discourse/routes/topic-from-params.js.es6

@@ -52,6 +52,7 @@ export default Discourse.Route.extend({
           enteredAt: new Date().getTime().toString()
         });
 
+        this.appEvents.trigger("page:topic-loaded", topic);
         topicController.subscribe();
 
         // Highlight our post after the next render