separate service deserialization from validation

Marshmallow validates and deserialises - BUT, when it deserialises,
it explicitly sets `sms_sender=None`, even when you haven't passed
sms_sender in. This is problematic, because we wanted to take advantage
of sqlalchemy's default value to set sms_sender to `GOVUK` when the
actual DB commit happens.

Instead, still use marshmallow for validating, but manually carry out
the json deserialisation in the model class.

This fixes a bug that only manifested when the database was upgraded,
but the code hadn't updated. 🎉

tests/app/service/test_rest.py

@@ -20,7 +20,7 @@ from tests.app.conftest import (
     sample_notification_history as create_notification_history,
     sample_notification_with_job
 )
-from app.models import KEY_TYPE_NORMAL, KEY_TYPE_TEAM, KEY_TYPE_TEST
+from app.models import Service, KEY_TYPE_NORMAL, KEY_TYPE_TEAM, KEY_TYPE_TEST
 
 from tests.app.db import create_user
 
@@ -216,6 +216,10 @@ def test_create_service(client, sample_user):
     assert json_resp['data']['dvla_organisation'] == '001'
     assert json_resp['data']['sms_sender'] == current_app.config['FROM_NUMBER']
 
+    service_db = Service.query.get(json_resp['data']['id'])
+    assert service_db.name == 'created service'
+    assert service_db.sms_sender == current_app.config['FROM_NUMBER']
+
     auth_header_fetch = create_authorization_header()
 
     resp = client.get(