Collate INTEGRAL update events into single event

Looking at some recent INTEGRAL events, it seems that each update notice for the same GCN event creates a new external event in GraceDB, where they should be appending the initial event: https://gracedb.ligo.org/latest/?query=INTEGRAL+1301764306.92+..+1339678000.98&query_type=E&get_neighbors=&results_format=

This seems to be because the trigger IDs are all different, which is how gwcelery/GraceDB determines whether an event is new or not. A solution could be to get the trigger ID from the IVORN address, which seems to be consistent for all the events.