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/config.py

@@ -135,6 +135,12 @@ class Config(object):
 
     CHECK_PROXY_HEADER = False
 
+    # these should always add up to 100%
+    SMS_PROVIDER_RESTING_POINTS = {
+        'mmg': 60,
+        'firetext': 40
+    }
+
     NOTIFY_SERVICE_ID = 'd6aa2c68-a2d9-4437-ab19-3ae8eb202553'
     NOTIFY_USER_ID = '6af522d0-2915-4e52-83a3-3690455a5fe6'
     INVITATION_EMAIL_TEMPLATE_ID = '4f46df42-f795-4cc4-83bb-65ca312f49cc'
@@ -198,6 +204,11 @@ class Config(object):
             'schedule': crontab(),
             'options': {'queue': QueueNames.PERIODIC}
         },
+        'tend-providers-back-to-middle': {
+            'task': 'tend-providers-back-to-middle',
+            'schedule': crontab(minute='*/5'),
+            'options': {'queue': QueueNames.PERIODIC}
+        },
         'check-for-missing-rows-in-completed-jobs': {
             'task': 'check-for-missing-rows-in-completed-jobs',
             'schedule': crontab(minute='*/10'),