Add notification center

Implement a notification center, e.g. using `swift-nio` cron jobs on the thread's current event loop. This would enable a range of scheduling-based features, like for example:

- [ ] Notifications for special days, e.g. International Talk Like A Pirate Day or user's "cake days" (their join dates)
- [ ] More streamlined representation of timers, which currently run on GCD timers
- [ ] More streamlined representation of countdowns (possibly even using features like ping subscriptions?)