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