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/notifications/notifications_letter_callback.py

@@ -13,7 +13,7 @@ from app.celery.tasks import update_letter_notifications_statuses
 from app.v2.errors import register_errors
 from app.notifications.utils import autoconfirm_subscription
 from app.schema_validation import validate
-from app.celery import QueueNames
+from app.config import QueueNames
 
 letter_callback_blueprint = Blueprint('notifications_letter_callback', __name__)
 register_errors(letter_callback_blueprint)

app/notifications/process_notifications.py

@@ -12,7 +12,7 @@ from app import redis_store
 from app.celery import provider_tasks
 from notifications_utils.clients import redis
 
-from app.celery import QueueNames
+from app.config import QueueNames
 from app.models import SMS_TYPE, Notification, KEY_TYPE_TEST, EMAIL_TYPE, ScheduledNotification
 from app.dao.notifications_dao import (dao_create_notification,
                                        dao_delete_notifications_and_history_by_id,

app/notifications/receive_notifications.py

@@ -6,7 +6,7 @@ from notifications_utils.recipients import validate_and_format_phone_number
 
 from app import statsd_client, firetext_client, mmg_client
 from app.celery import tasks
-from app.celery import QueueNames
+from app.config import QueueNames
 from app.dao.services_dao import dao_fetch_services_by_sms_sender
 from app.dao.inbound_sms_dao import dao_create_inbound_sms
 from app.models import InboundSms, INBOUND_SMS_TYPE, SMS_TYPE

app/notifications/rest.py

@@ -6,7 +6,7 @@ from flask import (
 )
 
 from app import api_user, authenticated_service
-from app.celery import QueueNames
+from app.config import QueueNames
 from app.dao import (
     templates_dao,
     notifications_dao