Convert string references to ::class

PHP 5.5.9 adds the new static `class` property which provides the fully qualified class name. This is preferred over using strings for class names since the `class` property references are checked by PHP.

app/Model/helpdesk/Agent_panel/Organization.php

@@ -15,7 +15,7 @@ class Organization extends BaseModel
 
     public function userRelation()
     {
-        $related = "App\Model\helpdesk\Agent_panel\User_org";
+        $related = \App\Model\helpdesk\Agent_panel\User_org::class;
 
         return $this->hasMany($related, 'org_id');
     }