Allow updates to SMS prefixing setting

We’re extracting this from being determined based on what the sender
name is to its own setting.

This commit will let users set it independently.

Until the explicitly set it, it will still be determined based on
whether their default sender name matches the default for the platform.

tests/__init__.py

@@ -56,7 +56,7 @@ def service_json(
     permissions=None,
     organisation_type='central',
     free_sms_fragment_limit=250000,
-    prefix_sms_with_service_name=None,
+    prefix_sms_with_service_name='Treat as None',
 ):
     if users is None:
         users = []
@@ -64,7 +64,7 @@ def service_json(
         permissions = ['email', 'sms']
     if inbound_api is None:
         inbound_api = []
-    if prefix_sms_with_service_name is None:
+    if prefix_sms_with_service_name == 'Treat as None':
         prefix_sms_with_service_name = (sms_sender == 'GOVUK')
     return {
         'id': id_,