Switch to listen to external events primarily over Kafka

Currently we listen to the majority of external events via GCN classic, which has numerous issues and will eventually be phased out to be superseded by the GCN Kafka system. See: https://gcn.nasa.gov/

This could be done in two phases:

1. Switch to from GCN classic to GCN classic over Kafka, which won't require any ingestion changes

2. Once GCN Kafka (JSON format) is ready, change ingestion methods over. This second phase will require the following changes:

• Ingest JSON natively in GraceDB: computing/gracedb/server#297
• Change workflow in external_triggers.py to use JSON packets when ingesting