Convert the rest of the API to new action architecture

Also make some tweaks:
- Merge SerializeAction::$include and
SerializeAction::$includeAvailable into a keyed boolean array
- Set defaults for SerializeAction::$limit and $limitMax
- Rename SerializeAction::$sortAvailable to $sortFields

src/Api/Actions/Posts/GetsPosts.php

@@ -1,6 +1,5 @@
 <?php namespace Flarum\Api\Actions\Posts;
 
-use Flarum\Core\Models\User;
 use Flarum\Api\JsonApiRequest;
 
 trait GetsPosts