Add page to change SMS allowance

There may be exceptions to the rule, so we should have a way of
overriding this manually.

app/notify_client/service_api_client.py

@@ -104,7 +104,9 @@ class ServiceAPIClient(NotifyAdminAPIClient):
             'organisation',
             'letter_contact_block',
             'dvla_organisation',
-            'permissions'
+            'permissions',
+            'organisation_type',
+            'free_sms_fragment_limit',
         }
         if disallowed_attributes:
             raise TypeError('Not allowed to update service attributes: {}'.format(