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.
* remove from model
* still required when calling POST /service - we just call through
from dao_create_service to add a new annual billing entry.
* removed from POST /service/<id> update_service - if you want to
update/add a new one, use POST /service/<id>/free-sms-fragment-limit
* made sure tests create services with default 250k limit.
* unused variables
* variables in loops overshadowing imports
* excepts with no defined exc type (tried to avoid `except Exception` too)
* history mapper is still too complex
* default variables should never be mutable
- 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
