Add a column for a service to set SMS prefixing

In future changes, services will be able to control whether their text
messages will be prefixed with the name of their service.

This commit:
- adds a column to store the value of that setting
- makes the service model take notice of it, if it were to have a value
  set

It doesn’t:
- provide a way of setting the value of this column

Currently the column can have three values:
- `None` – ignore it (this is what all current services will start as)
  and continue to determine whether to prefix messages by looking at the
  sender
- `True` – always the service name to the start of text messages
- `False` – never add the service name to the start of text messages

In the future we’ll migrate all services to be either `True` or `False`,
the `None` will go away and all services will have direct control over
the setting.

app/delivery/send_to_providers.py

@@ -48,7 +48,7 @@ def send_sms_to_provider(notification):
         )
         template_model = dao_get_template_by_id(notification.template_id, notification.template_version)
 
-        sender_has_been_customised = (not service.prefix_sms_with_service_name())
+        sender_has_been_customised = (not service.get_prefix_sms_with_service_name())
 
         template = SMSMessageTemplate(
             template_model.__dict__,