Use Celery instead of RQ.

Celery provides a more robust set of queueing options for both tasks and
worker processes. Updates include:
- infrastructure necessary to run Celery, including celery entrypoint
- backgrounded functions are now imported directly from atst.jobs
- update tests as-needed
- update kubernetes worker pod command

deploy/azure/azure.yml

@@ -143,9 +143,10 @@ spec:
           image: pwatat.azurecr.io/atat:884d95ada21a5097f5c07f305d8e4e24d0f2a03f
           args: [
             "/opt/atat/atst/.venv/bin/python",
-            "/opt/atat/atst/.venv/bin/flask",
-            "rq",
-            "worker"
+            "/opt/atat/atst/.venv/bin/celery",
+            "-A",
+            "celery_worker.celery",
+            "worker",
           ]
           resources:
             requests: