New strategy for transaction management.

Introduce a contextmanger function to handle exceptions and nested
transactions. Using the nested_transaction will start a
nested transaction with `db.session.begin_nested`, once the nested
transaction is complete the commit will happen.
`@transactional` has been updated to commit unless in a nested
transaction.

app/dao/organisation_dao.py

@@ -1,12 +1,7 @@
 from sqlalchemy.sql.expression import func
 
 from app import db
-from app.dao.dao_utils import (
-    VersionOptions,
-    nested_transactional,
-    transactional,
-    version_class,
-)
+from app.dao.dao_utils import VersionOptions, transactional, version_class
 from app.models import Domain, Organisation, Service, User
 
 
@@ -110,7 +105,7 @@ def _update_organisation_services(organisation, attribute, only_where_none=True)
         db.session.add(service)
 
 
-@nested_transactional
+@transactional
 @version_class(Service)
 def dao_add_service_to_organisation(service, organisation_id):
     organisation = Organisation.query.filter_by(