Login problem since RN 0.59

### Backgrounds

The current designed login process,

1. In login screen, visit Github's login url with a callback schema by WebView and listen to `onNavigationStateChange`
- Before #323, use `Linking` in Android and `react-native-safari-view` in iOS, then listen on Linking in both platforms
- Why still listening on Linking in Android? Because in Android, we have a `can't login` button
2. If succeed to login, it will,
1. jump to the schema, `gitpoint://welcome`, which means the welcome screen
2. trigger onNavigationStateChange to get the auth, and navigate to `Main`

I guess the stack navigator guarantees the main screen will always on the top of the welcome screen.

### Problem

I tested different versions of codes and found that starting from #883, with RN 0.59,

- In iOS, onNavigationStateChange is not triggered for the schema
- In Android, every onNavigationStateChange is triggered but Main can't cover Welcome

Users will be blocked by the welcome screen after login in. The workaround is killing the app and open again. It has logged in so will navigate to the main screen at the second time.

### Related

- No tag for v1.6.0, according to translation texts, it should be > 1716618 and < ecb61fa
- The most likely commit is 67a17ae
- Used try to make `Welcome` be able to navigate to `Main`(35e4a08)
- Works well in iOS, it will only try to auth in the welcome screen
- But in Android, it will try to auth twice (both in login and welcome screen)

Future direction should be replacing the WebView login with native implementations.

まず #883 周辺の変更とコミット 67a17ae、35e4a08 を確認し、その後 React Native 0.59 を使用して iOS と Android でログインフローを再現します。ログインが成功してアプリの再起動を必要とせずに Main に到達し、認証が二重に実行されなければ完了です。

索引モデルが issue の本文から書いたものです。