Move react-native template

Summary: This diff moves the React Native template from `local-cli/templates/HelloWorld` to `template`. Keeping it in the react-native repo will allow us to make changes to the template and version it together with react-native, instead of moving it out into a separate repo and trying to keep the template in sync with RN.

Reviewed By: TheSavior

Differential Revision: D13372949

fbshipit-source-id: e68e267b4dcf6384535d7b48fc11e297a12e9676