Implemented the rate limiting from Redis

- Uses Redis cache to check for current count
- If not present then sets the value based on the database state
- Any Redis errors are swallowed. Cache failures should NOT fail  the request.

app/notifications/rest.py

@@ -206,7 +206,6 @@ def get_notification_statistics_for_day():
 
 @notifications.route('/notifications/<string:notification_type>', methods=['POST'])
 def send_notification(notification_type):
-    redis_store.set('key1', 'value')
 
     if notification_type not in ['sms', 'email']:
         assert False