We already filter the usage-by-month query by financial year. When we
show the total usage for a service, we should be able to filter this
by financial year.
Then, when the two lots of data are put side by side, it all adds up.
- again these new come from the notifications history table
- We update this when we sent a notification, so removed from celery tasks
- tests removed also
use NotficationHistory instead. Unfortunately this means the SQL
gets a bit gnarly, as we have to repeat notifications_utils'
`get_sms_fragment_count` functionality inside a SELECT 😱