need public alert FAR criteria applied to multi-pipeline GCN circulars

Currently, upon a trigger landing in gracedb and meeting the public alert criteria, the auto-generated GCN Circulars include in it all the pipelines that have present a trigger in the corresponding Superevent. This is done independently of the FAR that such trigger has. This may lead to misleading conclusions one may draw by the mere coincidence of such multiple pipelines. WE will need to have some threshold (in FAR) for such inclusion in the GCN Circular; probably the very threshold for public alerts (correspondingly for bursts and CBC events).

Originally reported as gwcelery#156 (closed).