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/__init__.py

@@ -49,6 +49,7 @@ def create_app(app_name=None):
     from app.config import configs
 
     notify_environment = os.environ['NOTIFY_ENVIRONMENT']
+
     application.config.from_object(configs[notify_environment])
 
     if app_name: