make history meta handle nullable and default better

* can now handle nullable foreign keys - would previously raise
  AttributeError
* can now handle default values - we would previously not insert
  default values that hadn't been generated yet, which if the
  field is not nullable will result in IntegrityErrors. We were
  deliberately removing 'default' attributes from columns. Only
  remove them if nullable is set to true now.

tests/app/dao/test_services_dao.py

@@ -35,7 +35,8 @@ from app.models import (
     Permission,
     User,
     InvitedUser,
-    Service
+    Service,
+    BRANDING_GOVUK
 )
 
 from tests.app.conftest import (
@@ -60,10 +61,13 @@ def test_create_service(sample_user):
                       created_by=sample_user)
     dao_create_service(service, sample_user)
     assert Service.query.count() == 1
-    assert Service.query.first().name == "service_name"
-    assert Service.query.first().id == service.id
-    assert not Service.query.first().research_mode
-    assert sample_user in Service.query.first().users
+
+    service_db = Service.query.first()
+    assert service_db.name == "service_name"
+    assert service_db.id == service.id
+    assert service_db.branding == BRANDING_GOVUK
+    assert not service_db.research_mode
+    assert sample_user in service_db.users
 
 
 def test_cannot_create_two_services_with_same_name(sample_user):
@@ -254,6 +258,8 @@ def test_create_service_creates_a_history_record_with_current_data(sample_user):
     assert service_from_db.version == service_history.version
     assert sample_user.id == service_history.created_by_id
     assert service_from_db.created_by.id == service_history.created_by_id
+    assert service_from_db.branding == BRANDING_GOVUK
+    assert service_history.branding == BRANDING_GOVUK
 
 
 def test_update_service_creates_a_history_record_with_current_data(sample_user):