copy Nuclide token

Summary:
* We are now using `flipper.oAuthToken` to store our token in the keychain
* If there is no Flipper token, we copy the Nuclide token (once)
* If the user signs out of Flipper, only the flipper token is removed from the keychain, not the nuclide token
* the Nuclide token is not copied anymore

Reviewed By: passy

Differential Revision: D14598658

fbshipit-source-id: ab0646b35575bd4b2e433d62bb98635f6d127fd2

src/fb-stubs/user.js

@@ -37,3 +37,7 @@ export async function shareFlipperData(
   new window.Notification('Feature not implemented');
   return Promise.reject();
 }
+
+export async function writeKeychain(token: string) {
+  return Promise.reject();
+}