move monthly notification stats from notification history to ft tables

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

tests/app/service/test_rest.py

@@ -1481,36 +1481,18 @@ def test_get_detailed_service(notify_db, notify_db_session, notify_api, sample_s
     assert service['statistics'][SMS_TYPE] == stats
 
 
-@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):
-    mocker.patch(
-        'app.service.rest.dao_fetch_monthly_historical_stats_for_service',
-        return_value={'foo': 'bar'},
+@pytest.mark.parametrize('kwargs, expected_json', [
+    ({'year': 'baz'}, {'message': 'Year must be a number', 'result': 'error'}),
+    ({}, {'message': 'Year must be a number', 'result': 'error'}),
+])
+def test_get_monthly_notification_stats_returns_errors(admin_request, sample_service, kwargs, expected_json):
+    response = admin_request.get(
+        'service.get_monthly_notification_stats',
+        service_id=sample_service.id,
+        _expected_status=400,
+        **kwargs
     )
-    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
+    assert response == expected_json
 
 
 def test_get_services_with_detailed_flag(client, notify_db, notify_db_session):