Bugfix: Logout deleted user on page reload

A user that gets disabled or deleted in the backend now sees an authentication error page upon page reload.
From there they can now properly reach the login page to log in again via a different user (or leave the page entirely).

https://github.com/owncloud/web/issues/4677
https://github.com/owncloud/web/issues/4564
https://github.com/owncloud/web/issues/4795
https://github.com/owncloud/web/pull/7072
