FIX: Ensure slugless topic URLs are correctly redirected (#24719)

Using `DiscourseURL.routeTo` with `replaceURL: true` wouldn't cause a true Ember redirect. That meant that the transition source would be be replaced in the browser's history stack, and the 'back' button wouldn't work as expected. Instead, we can use the router service to perform a proper redirect.

app/assets/javascripts/discourse/app/routes/topic.js

@@ -355,12 +355,8 @@ const TopicRoute = DiscourseRoute.extend({
 
   model(params, transition) {
     if (params.slug.match(ID_CONSTRAINT)) {
-      transition.abort();
-
-      DiscourseURL.routeTo(`/t/topic/${params.slug}/${params.id}`, {
-        replaceURL: true,
-      });
-
+      // URL with no slug - redirect to a URL with placeholder slug
+      this.router.transitionTo(`/t/-/${params.slug}/${params.id}`);
       return;
     }