Add new event subtask.create_update

app/Core/Event/EventManager.php

@@ -3,6 +3,7 @@
 namespace Kanboard\Core\Event;
 
 use Kanboard\Model\TaskModel;
+use Kanboard\Model\SubtaskModel;
 use Kanboard\Model\TaskLinkModel;
 
 /**
@@ -54,6 +55,7 @@ class EventManager
             TaskModel::EVENT_ASSIGNEE_CHANGE   => t('Task assignee change'),
             TaskModel::EVENT_DAILY_CRONJOB     => t('Daily background job for tasks'),
             TaskModel::EVENT_MOVE_SWIMLANE     => t('Move a task to another swimlane'),
+            SubtaskModel::EVENT_CREATE_UPDATE  => t('Subtask creation or modification'),
         );
 
         $events = array_merge($events, $this->events);