Add prefix SMS with service name to service model

If the service is sending messages from GOVUK, then its messages should
be prefixed with the service name. Right now this logic is:
- worked out separately in the admin app and API
- isn’t aware of multiple senders

This commit moves the logic to one place (the service model). It does
this in a slightly naive way, in that it only looks at the default
sender, not the actual sender of the message.

In the future this will go away because we’ll move it to being a setting
that’s controlled independently of the service name. But this is the
first step towards that.

fixup! Add prefix SMS with service name to service model

tests/app/service/test_rest.py

@@ -156,6 +156,7 @@ def test_get_service_by_id(client, sample_service):
     assert json_resp['data']['branding'] == 'govuk'
     assert json_resp['data']['dvla_organisation'] == '001'
     assert json_resp['data']['sms_sender'] == current_app.config['FROM_NUMBER']
+    assert json_resp['data']['prefix_sms_with_service_name'] is True
 
 
 def test_get_service_by_id_returns_free_sms_limit(client, sample_service):
@@ -1561,6 +1562,36 @@ def test_set_sms_sender_for_service_rejects_null(client, sample_service):
     assert result['message'] == {'sms_sender': ['Field may not be null.']}
 
 
+@pytest.mark.parametrize('default_sms_sender, should_prefix', [
+    (None, True),  # None means use default
+    ('Foo', False),
+])
+def test_prefixing_messages_based_on_sms_sender(
+    client,
+    notify_db_session,
+    default_sms_sender,
+    should_prefix,
+):
+    service = create_service(
+        sms_sender=default_sms_sender or current_app.config['FROM_NUMBER']
+    )
+    create_service_sms_sender(
+        service=service,
+        sms_sender='ignored',
+        is_default=False,
+    )
+
+    result = client.get(
+        url_for(
+            'service.get_service_by_id',
+            service_id=service.id
+        ),
+        headers=[('Content-Type', 'application/json'), create_authorization_header()]
+    )
+    service = json.loads(result.get_data(as_text=True))['data']
+    assert service['prefix_sms_with_service_name'] == should_prefix
+
+
 @pytest.mark.parametrize('today_only,stats', [
     ('False', {'requested': 2, 'delivered': 1, 'failed': 0}),
     ('True', {'requested': 1, 'delivered': 0, 'failed': 0})