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/DiscussionSerializer.php

@@ -19,7 +19,7 @@ class DiscussionSerializer extends DiscussionBasicSerializer
     {
         $attributes = parent::attributes($discussion);
 
-        $user = static::$actor->getUser();
+        $user = $this->actor->getUser();
         $state = $discussion->stateFor($user);
 
         $attributes += [