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/JsonApiResponse.php

@@ -0,0 +1,19 @@
+<?php namespace Flarum\Api;
+
+use Illuminate\Http\Response;
+use Tobscure\JsonApi\Document;
+
+class JsonApiResponse extends Response
+{
+    public $content;
+
+    public function __construct($data = null, $status = 200, array $headers = [])
+    {
+        parent::__construct('', $status, $headers);
+
+        $this->headers->set('Content-Type', 'application/vnd.api+json');
+
+        $this->content = new Document;
+        $this->content->setData($data);
+    }
+}