Set task status to invalid instead of deleting

[MAILPOET-3359]

lib/Entities/ScheduledTaskEntity.php

@@ -17,6 +17,7 @@ class ScheduledTaskEntity {
   const STATUS_COMPLETED = 'completed';
   const STATUS_SCHEDULED = 'scheduled';
   const STATUS_PAUSED = 'paused';
+  const STATUS_INVALID = 'invalid';
   const VIRTUAL_STATUS_RUNNING = 'running'; // For historical reasons this is stored as null in DB
   const PRIORITY_HIGH = 1;
   const PRIORITY_MEDIUM = 5;