Add endpoint to get billable units/financial year

`/services/ef7a665d-11a4-425a-a180-a67ca00b69d7/billable-units?year=2016`

Pretty much just passes through to the DAO layer. Validates that year
is:

- present (there’s no need for unbounded queries on this endpoint)
- an integer

app/service/rest.py

@@ -310,3 +310,13 @@ def update_whitelist(service_id):
     else:
         dao_add_and_commit_whitelisted_contacts(whitelist_objs)
         return '', 204
+
+
+@service_blueprint.route('/<uuid:service_id>/billable-units')
+def get_billable_unit_count(service_id):
+    try:
+        return jsonify(notifications_dao.get_notification_billable_unit_count_per_month(
+            service_id, int(request.args.get('year'))
+        ))
+    except TypeError:
+        return jsonify(result='error', message='No valid year provided'), 400

tests/app/service/test_rest.py

@@ -1300,3 +1300,27 @@ def test_get_detailed_services_only_includes_todays_notifications(notify_db, not
         'email': {'delivered': 0, 'failed': 0, 'requested': 0},
         'sms': {'delivered': 0, 'failed': 0, 'requested': 2}
     }
+
+
+@freeze_time('2012-12-12T12:00:01')
+def test_get_notification_billable_unit_count(client, notify_db, notify_db_session):
+    notification = create_sample_notification(notify_db, notify_db_session)
+    response = client.get(
+        '/service/{}/billable-units?year=2012'.format(notification.service_id),
+        headers=[create_authorization_header(service_id=notification.service_id)]
+    )
+    assert response.status_code == 200
+    assert json.loads(response.get_data(as_text=True)) == {
+        'December': 1
+    }
+
+
+def test_get_notification_billable_unit_count_missing_year(client, sample_service):
+    response = client.get(
+        '/service/{}/billable-units'.format(sample_service.id),
+        headers=[create_authorization_header(service_id=sample_service.id)]
+    )
+    assert response.status_code == 400
+    assert json.loads(response.get_data(as_text=True)) == {
+        'message': 'No valid year provided', 'result': 'error'
+    }