closes https://github.com/TryGhost/Ghost/issues/10995
- when first loading the site preview, if private mode is enabled submit the login form in the background to get the cookie before loading the iframe
- refactors post-authentication preloading to ensure it occurs before post-authentication route hooks are called
- adds `showSuccess` attribute to `<GhTaskButton>` so that when set to `false` it can stay in the running state after "success" to avoid state change flashes whilst waiting for a transition
no issue
- `/config/` can only be requested when authenticated
- updated `/config/` mock to look for an Authentication header and return a 403 if it's missing
- updated `ajax` service to add an `Authentication` header when authenticated in testing env (cookies are not present when testing)
- updated `config` service to add `fetchUnauthenticated()` and `fetchAuthenticated()` methods in addition to `.fetch()`
- updated `application` route to only fetch authenticated config when authenticated
- updated `signin` controller to correctly fetch config after sign-in
