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/billing/rest.py

@@ -1,5 +1,6 @@
 from flask import Blueprint, jsonify, request
 
+from app import db
 from app.billing.billing_schemas import (
     create_or_update_free_sms_fragment_limit_schema,
     serialize_ft_billing_remove_emails,
@@ -83,6 +84,7 @@ def get_free_sms_fragment_limit(service_id):
                 annual_billing = dao_create_or_update_annual_billing_for_year(service_id,
                                                                               annual_billing.free_sms_fragment_limit,
                                                                               financial_year_start)
+                db.session.commit()
 
     return jsonify(annual_billing.serialize_free_sms_items()), 200
 
@@ -118,3 +120,4 @@ def update_free_sms_fragment_limit_data(service_id, free_sms_fragment_limit, fin
             free_sms_fragment_limit,
             financial_year_start
         )
+    db.session.commit()