🐛 Fixed error after logging in from a nested admin URL

closes https://github.com/TryGhost/Ghost/issues/10629
- the error was occurring due to `session.user` CP being populated with a rejected promise when attempting to access the first route. The CP has no dependent key so any further attempts to access `session.user` would be rejected
- marking the CP as "changed" immediately after logging in means that the next request will create a new promise and successfully fetch the user

ghost/admin/app/services/session.js

@@ -13,6 +13,9 @@ export default SessionService.extend({
     }),
 
     authenticate() {
+        // ensure any cached this.user value is removed and re-fetched
+        this.notifyPropertyChange('user');
+
         return this._super(...arguments).then((authResult) => {
             // TODO: remove duplication with application.afterModel
             let preloadPromises = [