Handle case where SubGRB is being appended to by threshold GRB

In the case of https://gracedb-test.ligo.org/events/E1178004/view/, we have an originally SubGRB event that was appended by a treshold GRB FINAL_NOTICE . Since the search was not updated from SubGRB , the even tried to get a sky map link from a non-existing field (the VOEvent templates don't match) and failed: https://ligo-caltech.sentry.io/issues/5998815303/?project=1425216

Currently, it is not clear how rare this is as only one instance of this happening has been recorded.

Possible solutions:

• Develop method with GraceDB developers to be able to change search field of event later.
• Create whole new external event with same TriggerID. Note that any further appending due to later updates will fail since there are multiple events with the same TriggerID.