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/invite/rest.py

@@ -4,7 +4,7 @@ from flask import (
     jsonify,
     current_app)
 
-from app.celery import QueueNames
+from app.config import QueueNames
 from app.dao.invited_user_dao import (
     save_invited_user,
     get_invited_user,