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/schemas.py

@@ -203,7 +203,7 @@ class ServiceSchema(BaseSchema):
         return service.get_default_letter_contact()
 
     def get_prefix_sms_with_service_name(self, service):
-        return service.prefix_sms_with_service_name()
+        return service.get_prefix_sms_with_service_name()
 
     class Meta:
         model = models.Service