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/JsonApiRequest.php

@@ -0,0 +1,29 @@
+<?php namespace Flarum\Api;
+
+class JsonApiRequest extends Request
+{
+    /**
+     * @var array
+     */
+    public $sort;
+
+    /**
+     * @var array
+     */
+    public $include;
+
+    /**
+     * @var array
+     */
+    public $link;
+
+    /**
+     * @var int
+     */
+    public $limit;
+
+    /**
+     * @var int
+     */
+    public $offset;
+}