config: Add event signals to Celery for better task tracking

Author: blarghmatey

Procfile

@@ -1,5 +1,5 @@
 release: bash scripts/heroku-release-phase.sh
 web: bin/start-nginx bin/start-pgbouncer newrelic-admin run-program uwsgi uwsgi.ini
-worker: bin/start-pgbouncer newrelic-admin run-program celery -A open_discussions.celery:app worker -Q spam,default --concurrency=2 -B -l $OPEN_DISCUSSIONS_LOG_LEVEL
-extra_worker_2x: bin/start-pgbouncer newrelic-admin run-program celery -A open_discussions.celery:app worker -Q spam,digest_emails,edx_content,default --concurrency=2 -l $OPEN_DISCUSSIONS_LOG_LEVEL
-extra_worker_performance: bin/start-pgbouncer newrelic-admin run-program celery -A open_discussions.celery:app worker -Q spam,digest_emails,edx_content,default -l $OPEN_DISCUSSIONS_LOG_LEVEL
+worker: bin/start-pgbouncer newrelic-admin run-program celery -A open_discussions.celery:app worker -E -Q spam,default --concurrency=2 -B -l $OPEN_DISCUSSIONS_LOG_LEVEL
+extra_worker_2x: bin/start-pgbouncer newrelic-admin run-program celery -A open_discussions.celery:app worker -E -Q spam,digest_emails,edx_content,default --concurrency=2 -l $OPEN_DISCUSSIONS_LOG_LEVEL
+extra_worker_performance: bin/start-pgbouncer newrelic-admin run-program celery -A open_discussions.celery:app worker -E -Q spam,digest_emails,edx_content,default -l $OPEN_DISCUSSIONS_LOG_LEVEL