Allow filtering of billing info by financial year

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.

app/dao/provider_statistics_dao.py

@@ -8,15 +8,21 @@ from app.models import (
     NOTIFICATION_STATUS_TYPES_BILLABLE,
     KEY_TYPE_TEST
 )
+from app.dao.notifications_dao import get_financial_year
 
 
-def get_fragment_count(service_id):
+def get_fragment_count(service_id, year=None):
     shared_filters = [
         NotificationHistory.service_id == service_id,
         NotificationHistory.status.in_(NOTIFICATION_STATUS_TYPES_BILLABLE),
         NotificationHistory.key_type != KEY_TYPE_TEST
     ]
 
+    if year:
+        shared_filters.append(NotificationHistory.created_at.between(
+            *get_financial_year(year)
+        ))
+
     sms_count = db.session.query(
         func.sum(NotificationHistory.billable_units)
     ).filter(