Fix OC_User::getDisplayName to persist between (browser) sessions. Fixes #3694

See #3694. "display_name" is only present in $_SESSION after setting it on the settings page. The session is lost after browser restart, only user_id is reloaded from the cookie.

This fix fetches the display name from the backend by using the user_id session variable.

(The whole `$_SESSION['display_name']` is pointless, but as the user backend is being rewritten for OC6, I won't mess around there)