remove PHP generated user list and move all to the existing JS part

I cleaned up the PHP part of the user management. The whole user list is now generated via the existing JS code.

* moved loading of JS and CSS to template

ref #12620

@blizzz @LukasReschke @PVince81 Give this a try and report back problems.

TODO:
* [x] changing groups cause problems with the group drop down
merge into the first call?^^