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.

tests/app/db.py

@@ -67,7 +67,8 @@ def create_service(
     research_mode=False,
     active=True,
     do_create_inbound_number=True,
-    email_from=None
+    email_from=None,
+    prefix_sms=None,
 ):
     service = Service(
         name=service_name,
@@ -76,6 +77,7 @@ def create_service(
         email_from=email_from if email_from else service_name.lower().replace(' ', '.'),
         created_by=user or create_user(email='{}@digital.cabinet-office.gov.uk'.format(uuid.uuid4())),
         sms_sender=sms_sender,
+        prefix_sms=prefix_sms,
     )
 
     dao_create_service(service, service.created_by, service_id, service_permissions=service_permissions)