Revert celery4

Revert the following three pull requests:
https://github.com/alphagov/notifications-api/pull/1085
https://github.com/alphagov/notifications-api/pull/1086
https://github.com/alphagov/notifications-api/pull/1088

celery 4.0.2 looked promising, however, on staging under mild load
(5/sec api calls) the performance was actually worse than 3.1.25

app/delivery/rest.py

@@ -1,6 +1,6 @@
 from flask import Blueprint, jsonify
 
-from app.celery import QueueNames
+from app.config import QueueNames
 from app.delivery import send_to_providers
 from app.models import EMAIL_TYPE
 from app.celery import provider_tasks