Use responsive datatables in Users view: automatic column priority etc.

Use responsive datatables in Users view: automatic column priority etc.
* Actually in mobile this seems to have a glitch, it shows one column too
  much, which corrects itself as soon as you scroll.
* On a big screen the "Oper" and "Secure" columns are still not shown
  even though they could be. Then again, those columns are not really
  important so may be scratched altogether.
* If all this turns out not to work too well, then we can always revert
  revert the changes to users/index.php, i guess.