users: Show username hint immediately

Changes made by GNOME/gnome-control-center!359 caused that the username
hint ("This will be used to name your home folder and can’t be changed.")
is not shown immediately after opening the "Add User" dialog. This change
was unwanted. Let's show the hint immediately after opening the dialog as
it was before.

panels/user-accounts/cc-add-user-dialog.c

@@ -1577,6 +1577,7 @@ cc_add_user_dialog_init (CcAddUserDialog *self)
         self->local_password_mode = ACT_USER_PASSWORD_MODE_SET_AT_LOGIN;
         dialog_validate (self);
         update_password_strength (self);
+        local_username_timeout (self);
 
         enterprise_check_domain (self);