Prevent XSS via $_GET param for controller action

Originally fixed in 2.2.x via https://github.com/modxcms/revolution/commit/77463eb6a8090f474b04fdc1b72225cb93c558ea - was reverted somehow

Should address #11966
Better store check when trying to add a group to a user (from user panel)
Typecast user group
Fix front-end user group comparison bug when assigning a new user group

Resolves #11399

* upstream/pr/11975:
  Typecast user group
  Better store check when trying to add a group to a user (from user panel)