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/ApiServiceProvider.php

@@ -18,8 +18,6 @@ class ApiServiceProvider extends ServiceProvider
         );
 
         include __DIR__.'/routes.php';
-
-        BaseSerializer::setActor($this->app['Flarum\Support\Actor']);
     }
 
     /**