Implement preferred_event_is_offline key in superevent packet and lvalert
In retrospect, it would be useful for follow-up pipelines to know for
update superevent LVAlerts to know if the preferred event is online or offline. That would require one less query to GraceDB or one less listener LVAlert listener.
This should be pretty easy to implement since the superevent already inherits info from preferred event, so this would just be one more boolean to add to the LVALert.