Cleanup task for message_popup_notifications table

messaging_cleanup_task removes 'old' read and unread message notifications based on $CFG->messagingdeletereadnotificationsdelay and $CFG->messagingdeleteallnotificationsdelay settings. But doesn't remove any record from message_popup_notifications. So there are some orphaned records in message_popup_notifications table remaining with no related message_notifications record.

There should be a cleanup task to remove all orphaned records in message_popup_notifications table.