flow: Add 'Linking'-related types in 'AuthScreen'
flow: Add 'Linking'-related types in 'AuthScreen'

The `LinkingEvent` type does not exist explicitly in React Native's
code and was confirmed correct by reading the source code and looking
at the documentation in:
https://facebook.github.io/react-native/docs/linking

`getInitialURL` returns only `string` or `null` but is already
internally defined as `?string` in RN so use that instead.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile