Use BackgroundFetch and LocalNotifications to notify of new chapters

- [BackgroundFetch API](https://docs.expo.io/versions/v35.0.0/sdk/background-fetch/)
- [Local Notification API](https://docs.expo.io/versions/latest/sdk/notifications/#notificationspresentlocalnotificationasynclocalnotification)

There's no server component to this, so we can't do this server-side and send a push notification, have to do it client-side and send local notification. Of course the major downside to this (apart from battery and phone usage) is that if the app is terminated or the phone is restarted, the BackgroundFetch won't start until the app is opened again 😕 On Android, there is a workaround built-in for both these cases, but not for iOS.

Sending Local Notif is easy enough, but BackgroundFetch requires some more reading and a bit of effort to get familiar and set-up.

Related to this would be loading chapters on the front-page / main view and highlighting mangas which have new unread chapters. New Unread Chapter indicates highlight in-app and LocalNotification out-of-app (probably no need to notify if app is still open, user will probably see the new chapter indicator themselves... well not if they quit the app while inside a manga... idk).