Frontend extender: Route to correct frontend

So far, we always added routes configured via this extender to the
forum frontend. Not correct.

framework/core/src/Extend/Frontend.php

@@ -87,7 +87,7 @@ class Frontend implements ExtenderInterface
         foreach ($this->routes as $route) {
             $routes->get(
                 $route['path'], $route['name'],
-                $factory->toForum($route['content'])
+                $factory->toFrontend($this->frontend, $route['content'])
             );
         }
     }