Do not send notifications in Celery tasks if notifications are disabled

[giohappy]

There are two problems with the notifications in Celery tasks:

1. send_queued_notifications doesn't check if settings.NOTIFICATION_ENABLED is True. We already have geonode.notification_helpers.has_notifications for that, and when the module is imported it also automatically imports the module. The geonode.task.tasks module should import the helper module and use the has_notifications inside the send_queued_notifications method.
2. send_queued_notifications itself shouldn't be called if notifications are disabled. We must do this check inside the geonode.notification_helpers.has_notifications.call_celery decorator and avoid triggering the Celery task in case notifications are disabled.