Update API action architecture

- An API action handles a Flarum\Api\Request, which is a simple object
containing an array of params, the actor, and optionally an HTTP
request object
- Most API actions use SerializeAction as a base, which parses request
input according to the JSON-API spec (creates a JsonApiRequest object),
runs the child class method to get data, then serializes it and assigns
it to a JsonApiResponse (standard HTTP response with a
Tobscure\JsonApi\Document as content)
- The JSON-API request input parsing is subject to restrictions as
defined by public static properties on the action (i.e. extensible)
- Also the actor is given to the serializer instance now, instead of
being a static property

src/Api/Serializers/BaseSerializer.php

@@ -11,22 +11,13 @@ use Closure;
  */
 abstract class BaseSerializer extends SerializerAbstract
 {
-    /**
-     * The actor who is requesting the serialized objects.
-     *
-     * @var \Flarum\Support\Actor
-     */
-    protected static $actor;
+    protected $actor;
 
-    /**
-     * Set the actor who is requesting the serialized objects.
-     *
-     * @param  \Flarum\Support\Actor  $actor
-     * @return void
-     */
-    public static function setActor(Actor $actor)
+    public function __construct(Actor $actor, $include = null, $link = null)
     {
-        static::$actor = $actor;
+        parent::__construct($include, $link);
+
+        $this->actor = $actor;
     }
 
     /**
@@ -70,7 +61,7 @@ abstract class BaseSerializer extends SerializerAbstract
                 $class = get_class(is_object($data) ? $data : $model->$relation()->getRelated());
                 $serializer = $serializer[$class];
             }
-            $serializer = new $serializer($links);
+            $serializer = new $serializer($this->actor, $links);
             return $many ? $serializer->collection($data) : $serializer->resource($data);
         };
     }