Move react-native-starter into a separate repository

[anorsich]

The Stack repository should serve as a reference and main knowledge centre, all components should reside in distinct repositories.

1. Move react-native-starter into the separate repository.
2. Update readme according to guidelines. See node-mongo or landing starter for more details.
3. Update reference to the new repository in Stack readme.
4. Review our documentation, while setting up project for Startup Winter
5. Update to latest version of React Native