Adding @nested_transactional for transactions that require more than one

db update/insert.

Using a savepoint for the multiple transactions allows us to rollback if
there is an error when executing the second db transaction.
However, this does add a bit of complexity. Developers need to manage
the db session when calling multiple nested tranactions.

Unit tests have been added to test this functionality and some end to
end tests have been done to make sure all transactions are rollback if
there is an exception while executing the transaction.

app/dao/services_dao.py

@@ -7,7 +7,12 @@ from sqlalchemy.orm import joinedload
 from sqlalchemy.sql.expression import and_, asc, case, func
 
 from app import db
-from app.dao.dao_utils import VersionOptions, transactional, version_class
+from app.dao.dao_utils import (
+    VersionOptions,
+    nested_transactional,
+    transactional,
+    version_class,
+)
 from app.dao.date_util import get_current_financial_year
 from app.dao.email_branding_dao import dao_get_email_branding_by_name
 from app.dao.letter_branding_dao import dao_get_letter_branding_by_name
@@ -284,7 +289,7 @@ def dao_fetch_service_by_id_and_user(service_id, user_id):
     ).one()
 
 
-@transactional
+@nested_transactional
 @version_class(Service)
 def dao_create_service(
     service,