Add small site with a service worker, make MQTT optional, use HTTPS (will abandon)

- A small website registers a service worker and subscribes to push notifications
- MQTT is now optional, set `MQTT_ENABLED = 1` in settings.toml to opt in
- Configured the web server to use HTTPS. However, even with a small 1024-bit RSA certificate, performance is too slow (~10 seconds per response) and the service worker doesn't even load. Going to deploy this to the cloud instead eventually, and have the Pico just receive push subscriptions and send push notifications.