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)

tests/app/delivery/test_send_to_providers.py

@@ -722,6 +722,39 @@ def test_should_handle_sms_sender_and_prefix_message(
     )
 
 
+@pytest.mark.parametrize('sms_sender, prefix_setting, expected_content', [
+    ('foo', True, 'Sample service: bar'),
+    ('foo', False, 'bar'),
+    ('foo', None, 'bar'),
+    # 'testing' is the default SMS sender in unit tests
+    ('testing', None, 'Sample service: bar'),
+    ('testing', False, 'bar'),
+])
+def test_should_handle_sms_prefix_setting(
+    mocker,
+    sms_sender,
+    prefix_setting,
+    expected_content,
+    notify_db_session
+):
+    mocker.patch('app.mmg_client.send_sms')
+    mocker.patch('app.delivery.send_to_providers.create_initial_notification_statistic_tasks')
+    service = create_service(
+        sms_sender=sms_sender,
+        prefix_sms=prefix_setting,
+    )
+    template = create_template(service, content='bar')
+    notification = create_notification(template)
+
+    send_to_providers.send_sms_to_provider(notification)
+    mmg_client.send_sms.assert_called_once_with(
+        content=expected_content,
+        sender=ANY,
+        to=ANY,
+        reference=ANY,
+    )
+
+
 def test_should_use_inbound_number_as_sender_if_default_sms_sender(
         notify_db_session,
         mocker