add day filter to template usage dao, and remove NotificationHistory

we should be very careful with when we get data from
NotificationHistory - this should probably only be from scheduled
tasks. `dao_get_template_usage` is only called from the template
statistics rest endpoint, so shouldn't ever hit template history.

also, moved tests out to new file to break up the 2k test file a bit

app/template_statistics/rest.py

@@ -35,7 +35,7 @@ def get_template_statistics_for_service_by_day(service_id):
             message = {'limit_days': [error]}
             raise InvalidRequest(message, status_code=400)
     else:
-        limit_days = None
+        limit_days = 7
 
     if limit_days == 7:
         stats = get_template_statistics_for_7_days(limit_days, service_id)