Define static properties on SerializeAction subclasses

Explained in d1e7453ffd.

If we ever come up with a better way of doing this it should be easy to
change over, since modification of these properties by extensions is
abstracted by an Extend API.

src/Api/Actions/Posts/UpdateAction.php

@@ -14,12 +14,40 @@ class UpdateAction extends SerializeResourceAction
     protected $bus;
 
     /**
-     * The name of the serializer class to output results with.
-     *
-     * @var string
+     * @inheritdoc
      */
     public static $serializer = 'Flarum\Api\Serializers\PostSerializer';
 
+    /**
+     * @inheritdoc
+     */
+    public static $include = [];
+
+    /**
+     * @inheritdoc
+     */
+    public static $link = [];
+
+    /**
+     * @inheritdoc
+     */
+    public static $limitMax = 50;
+
+    /**
+     * @inheritdoc
+     */
+    public static $limit = 20;
+
+    /**
+     * @inheritdoc
+     */
+    public static $sortFields = [];
+
+    /**
+     * @inheritdoc
+     */
+    public static $sort;
+
     /**
      * Instantiate the action.
      *