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
refs #9865
- removed all `oauth2` and token-based ESA auth
- added new `cookie` authenticator which handles session creation
- updated the session store to extend from the `ephemeral` in-memory store and to restore by fetching the currently logged in user and using the success/failure state to indicate authentication state
- ESA automatically calls this `.restore()` method on app boot
- the `session` service caches the current-user query so there's no unnecessary requests being made for the "logged in" state
- removed the now-unnecessary token refresh and logout routines from the `application` route
- removed the now-unnecessary token refresh routines from the `ajax` service
- removed `access_token` query param from iframe file downloaders
- changed Ember Data adapters and `ghost-paths` to use the `/ghost/api/v2/admin/` namespace
