Fix ingestion of external events to playground

Since 2020-01-24 we haven't properly handled or uploaded an external event on playground from GCN (see: https://gracedb-playground.ligo.org/search/?query=External&query_type=E&results_format=S)

This is likely due to the commits 69be372e and/or 747bac6b made on 2020-01-24 and 2020-01-25 respectfully.