Design a Notification Service

Let's make an object-oriented design for a notification service, the part of a product that sends emails, text messages and push alerts.

Every product grows one of these. It starts as a function that sends an email. Then somebody wants text messages. Then somebody wants to stop being woken at three in the morning. Then marketing wants a limit so nobody gets five messages in an hour. Six months later that one function has fourteen conditions in it and nobody will touch it. This design is about where each of those additions should have gone instead.