Kafka alert generator

Addresses #388 (closed), and depends on !851 (merged).

Feb 11 edit: First attempt at adding the code to publish to a kafka topic is done, needs testing. Alerts currently set to publish to StandinKafkaTopic

Aug 10 edit: Ready to be reviewed. Need new release of hop-client for a PR needed for GCN support (merged) and need to update URL of avro schema files once userguide!124 (merged) is merged.

Sep 21 edit: Peeled a lot of these changes off into other MRs, specifically !914 (merged), !915 (merged), and !917 (merged). This MR now primarily deals with the work needed to plug the kafka alerts into the orchestrator.

To-Do list:

• Test sending avro packet to kafka producer
• Decide where to store avro schema files for deployed instances to read
• Write unit tests to...
  • Validate that avro packet constructed by voevent is correct
  • Check that avro packet is sent to kafka producer

gwcelery/tasks/igwn_alert.py

 """IGWN alert client."""
+import json
+
 from celery.utils.log import get_task_logger
 
 from ..igwn_alert.signals import igwn_alert_received
@@ -18,7 +20,12 @@ class _IGWNAlertDispatchHandler(DispatchHandler):
         igwn_alert_topics.update(keys)
         return super().__call__(*keys, **kwargs)
 
-    def process_args(self, topic, alert):
+    def process_args(self, topic, message):
+        # FIXME: Loading the message with json.loads() is a change required
+        # because we are using a newer version of hop-client than IGWN-Alert
+        # is, once IGWN-Alert upgrades to the newest version the message may
+        # become a dictionary once again
+        alert = json.loads(message.content)
         # Determine GraceDB service URL
         try:
             try:
@@ -28,7 +35,7 @@ class _IGWNAlertDispatchHandler(DispatchHandler):
         except KeyError:
             log.exception(
                 'IGWN alert message does not contain an API URL: %r',
-                alert)
+                message)
             return None, None, None
         base, api, _ = self_link.partition('/api/')
         service = base + api