Add push notifications for watched repos

It would be awesome to get push notifications for my watched repos. This is something that all other GitHub apps either don't have or are a paid feature.

I could definitely work on at least the iOS push notifications, though Android doesn't seem to be much different.

This might help: https://medium.com/differential/how-to-setup-push-notifications-in-react-native-ios-android-30ea0131355e

首先查看链接文章中描述的 React Native 推送通知设置，以及 GitPoint 中现有的受监视 repository 行为。定义受监视 repository 的事件应如何在 iOS 和 Android 上触发通知；完成标准是用户能够在两个平台上可靠地收到这些通知。

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