Launch subthreshold search from an external trigger based on pipeline
This is to fix a bug in !734 (merged) where we are launching subthreshold searches for both Fermi and Swift off of a single external trigger, which could itself either be from Fermi and Swift. See example: https://gracedb-test.ligo.org/events/E70592/view/
Instead we should launch one search using that one pipeline/time window appropriate for the external trigger.