We were already returning the month, notification_type, billing_units
and rate from the /monthly-usage billing endpoint. This adds in the
postage too so that we can display postage details on the usage page of
admin.
- Change the usage queries to a union so that billing_units is correct for all notification types. Removing the business logic from the schema.
- Added tests for different fragment counts, rates and sheet counts.
- 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