Avoid Celery/RabbitMQ "connection reset" errors [#5609].

This change starts Celery only after RabbitMQ is available. It also returns the
unnecessarily low BROKER_HEARTBEAT setting to default of 120.0 to help prevent
connections being reset on machines under load.

docs/source/siteadmin/deploying.rst

@@ -528,6 +528,7 @@ environment's setup:
     # Set the WorkingDirectory and Environment values to match your environment.
     [Unit]
     Description=MediaGoblin Celeryd
+    After=rabbitmq-server.service
 
     [Service]
     User=mediagoblin