it now uses the ft_notification_status table - nice and quick. if you
ask for the current tax year it'll top up the data with numbers from
the notification table.
the data is in exactly the same shape as the old endpoint - no changes
to the admin app are necessary
Added in a new endpoint and DAO function to provide the data for the new
platform admin statistics page. The DAO method gets different data from
the Notifications / NotificationHistory table and also groups it differently.
The old endpoint has not been deleted yet to allow the numbers on the
old and new pages to be compared.
moved format_statistics to a new service/statistics.py file, and
refactored to share code. moved tests as well, to try and enforce
separation between the restful endpoints of rest.py and the logic/
data manipulation of statistics.py
