Add endpoint for breakdown of activity by month

This endpoint will eventualy replace the weekly breakdown one. By month
for a given financial year is better, because it gives us consistency
with the breakdown of financial usage (and eventually consistency with
the template usage).

The code to do this is a bit convoluted, in order to fill out the counts
for months and statuses where we don’t have notifications.

This will make the admin side of this easier, because we can rely on
there always being numbers available. The admin side will deal with
summing the statuses (eg `temporary-failure` > `failed`) because this
is presentational.

This commit also modifies the usage count to use `.between()` for
consistency.

tests/app/service/test_rest.py

@@ -1221,6 +1221,38 @@ def test_get_weekly_notification_stats(notify_api, notify_db, notify_db_session)
     }
 
 
+@pytest.mark.parametrize(
+    'url, expected_status, expected_json', [
+        (
+            '/service/{}/notifications/monthly?year=2001',
+            200,
+            {'data': {'foo': 'bar'}},
+        ),
+        (
+            '/service/{}/notifications/monthly?year=baz',
+            400,
+            {'message': 'Year must be a number', 'result': 'error'},
+        ),
+        (
+            '/service/{}/notifications/monthly',
+            400,
+            {'message': 'Year must be a number', 'result': 'error'},
+        ),
+    ]
+)
+def test_get_monthly_notification_stats(mocker, client, sample_service, url, expected_status, expected_json):
+    mock_dao = mocker.patch(
+        'app.service.rest.dao_fetch_monthly_historical_stats_for_service',
+        return_value={'foo': 'bar'},
+    )
+    response = client.get(
+        url.format(sample_service.id),
+        headers=[create_authorization_header()]
+    )
+    assert response.status_code == expected_status
+    assert json.loads(response.get_data(as_text=True)) == expected_json
+
+
 def test_get_services_with_detailed_flag(notify_api, notify_db, notify_db_session):
     notifications = [
         create_sample_notification(notify_db, notify_db_session),