refactored billing/rest.py and annual_billing_dao.py to remove logic
from the dao, and simplify the process around creating new rows. Make
sure that the POST always creates (it previously wouldn't create rows
for years that don't already exist). Clean up some tests that were
doing too much set-up/data verification via rest calls rather than
directly inserting test data in to the DB.
- moved get_current_financial_year_start_year from service.utils to dao.date_utils
- Moved logic for data persistence from rest to dao when updating records in db
1. Create a separate billing blueprint to house these endpoints
2. Return monthly breakdown in same format as we did before
3. Return yearly breakdown but only return {billing units, rate,
notification_type}. Admin only makes use of these.
