add task to move sms providers back towards shared load

we generally aim to share the load between the two providers equally
(more or less). When one provider has struggled, we deprioritise them,
this commit adds a function that gradually restores balance. It checks
every five minutes, if it's been more than an hour since the providers
were last changed then it adjusts them towards a 50/50 split. Except
it's not quite 50/50 due to #reasons (we want to slightly favour MMG),
it's actually 60/40. That's defined in a new dict in config.py.

app/celery/scheduled_tasks.py

@@ -33,7 +33,10 @@ from app.dao.notifications_dao import (
     letters_missing_from_sending_bucket,
     is_delivery_slow_for_providers,
 )
-from app.dao.provider_details_dao import dao_reduce_sms_provider_priority
+from app.dao.provider_details_dao import (
+    dao_reduce_sms_provider_priority,
+    dao_adjust_provider_priority_back_to_resting_points
+)
 from app.dao.users_dao import delete_codes_older_created_more_than_a_day_ago
 from app.dao.services_dao import dao_find_services_sending_to_tv_numbers, dao_find_services_with_high_failure_rates
 from app.models import (
@@ -126,6 +129,12 @@ def switch_current_sms_provider_on_slow_delivery():
                 dao_reduce_sms_provider_priority(provider_name, time_threshold=timedelta(minutes=10))
 
 
+@notify_celery.task(name='tend-providers-back-to-middle')
+@statsd(namespace='tasks')
+def tend_providers_back_to_middle():
+    dao_adjust_provider_priority_back_to_resting_points()
+
+
 @notify_celery.task(name='check-job-status')
 @statsd(namespace="tasks")
 def check_job_status():