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/annual_billing_dao.py

@@ -1,12 +1,12 @@
 from flask import current_app
 
 from app import db
-from app.dao.dao_utils import transactional
+from app.dao.dao_utils import nested_transactional, transactional
 from app.dao.date_util import get_current_financial_year_start_year
 from app.models import AnnualBilling
 
 
-@transactional
+@nested_transactional
 def dao_create_or_update_annual_billing_for_year(service_id, free_sms_fragment_limit, financial_year_start):
     result = dao_get_free_sms_fragment_limit_for_year(service_id, financial_year_start)
 
@@ -53,7 +53,7 @@ def dao_get_all_free_sms_fragment_limit(service_id):
     ).order_by(AnnualBilling.financial_year_start).all()
 
 
-def set_default_free_allowance_for_service(service, year_start=None, commit=True):
+def set_default_free_allowance_for_service(service, year_start=None):
     default_free_sms_fragment_limits = {
         'central': {
             2020: 250_000,