Added Scheduled task to get stats for template usage

Currently some pages are timing out due to the time it takes to perform
database queries. This is an attempt to improve the performance by
performing the query against the notification history table once a day
and use the notification table for a delta between midnight and the when
the page is run and combine the results.

- Added Celery task for doing the work
- Added a dao to handle the insert and update of the stats table
- Updated tests to test the new functionality

tests/app/dao/test_services_dao.py

@@ -30,8 +30,8 @@ from app.dao.services_dao import (
     dao_suspend_service,
     dao_resume_service,
     dao_fetch_active_users_for_service,
-    dao_fetch_service_by_inbound_number
-)
+    dao_fetch_service_by_inbound_number,
+    dao_fetch_monthly_historical_stats_by_template_for_service_without_status)
 from app.dao.service_permissions_dao import dao_add_service_permission, dao_remove_service_permission
 from app.dao.users_dao import save_model_user
 from app.models import (
@@ -1004,3 +1004,38 @@ def _assert_service_permissions(service_permissions, expected):
 
     assert len(service_permissions) == len(expected)
     assert set(expected) == set(p.permission for p in service_permissions)
+
+
+def test_dao_fetch_monthly_historical_stats_by_template(notify_db, notify_db_session):
+    notification_history = functools.partial(
+        create_notification_history,
+        notify_db,
+        notify_db_session,
+        status='delivered'
+    )
+
+    template_one = create_sample_template(notify_db, notify_db_session)
+    template_two = create_sample_template(notify_db, notify_db_session)
+
+    notification_history(created_at=datetime(2017, 10, 1), sample_template=template_one)
+    notification_history(created_at=datetime(2016, 4, 1), sample_template=template_two)
+    notification_history(created_at=datetime(2016, 4, 1), sample_template=template_two)
+    notification_history(created_at=datetime.now(), sample_template=template_two)
+
+    results = dao_fetch_monthly_historical_stats_by_template_for_service_without_status()
+
+    assert len(results) == 2
+
+    for result in results:
+        if result.template_id == template_one.id:
+            assert result.template_id == template_one.id
+            assert result.month.month == 10
+            assert result.year.year == 2017
+            assert result.count == 1
+        elif result.template_id == template_two.id:
+            assert result.template_id == template_two.id
+            assert result.month.month == 4
+            assert result.year.year == 2016
+            assert result.count == 2
+        else:
+            raise AssertionError()