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/Actions/SerializeCollectionAction.php

@@ -0,0 +1,18 @@
+<?php namespace Flarum\Api\Actions;
+
+use Tobscure\JsonApi\SerializerInterface;
+
+abstract class SerializeCollectionAction extends SerializeAction
+{
+    /**
+     * Serialize the data as appropriate.
+     *
+     * @param \Tobscure\JsonApi\SerializerInterface $serializer
+     * @param array $data
+     * @return \Tobscure\JsonApi\Elements\Collection
+     */
+    protected function serialize(SerializerInterface $serializer, $data)
+    {
+        return $serializer->collection($data);
+    }
+}