Use ::class instead of some hardcoded class names

This gives us better refactoring functionality in IDEs like
PhpStorm, and also more quickly surfaces typos through errors
about undefined classes. :)

src/Api/Controller/ListPostsController.php

@@ -11,6 +11,7 @@
 
 namespace Flarum\Api\Controller;
 
+use Flarum\Api\Serializer\PostSerializer;
 use Flarum\Event\ConfigurePostsQuery;
 use Flarum\Post\PostRepository;
 use Illuminate\Database\Eloquent\Builder;
@@ -23,7 +24,7 @@ class ListPostsController extends AbstractListController
     /**
      * {@inheritdoc}
      */
-    public $serializer = 'Flarum\Api\Serializer\PostSerializer';
+    public $serializer = PostSerializer::class;
 
     /**
      * {@inheritdoc}