Show user backend in user management

* add switch to settings to show user backend
* user classes for headers and rows to unify the show/hide statement
* add user backend to response of user create request

ref #12604 and #12620

* [ ] requires #12790 to be merged first (included in this pr as well)

essential commit:  24895e3

cc @PVince81 @LukasReschke @blizzz