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/Request.php

@@ -0,0 +1,25 @@
+<?php namespace Flarum\Api;
+
+use Flarum\Support\Actor;
+use Illuminate\Http\Request as IlluminateRequest;
+
+class Request
+{
+    public $input;
+
+    public $actor;
+
+    public $httpRequest;
+
+    public function __construct(array $input, Actor $actor, IlluminateRequest $httpRequest = null)
+    {
+        $this->input = $input;
+        $this->actor = $actor;
+        $this->httpRequest = $httpRequest;
+    }
+
+    public function get($key, $default = null)
+    {
+        return isset($this->input[$key]) ? $this->input[$key] : $default;
+    }
+}