ADMIN : add "put in group" option when creating/editing users

[gvincke]

In ADMIN mode, when creating a new user, it would be very usefull to have a select boxes to be able to directly add the user in one or more groups without having to first edit/create it, and then open the group tab, find the user in the (sometime veeeeeeeeeeery loooooooong) select list, and then click on "add user". Thanks !

[geektortoise]

The group feature need to be refactored. I keep the ticket open (removing v2 milestone) as it will be used to design the refactoring.