Move User and Group displays into a macro

Move some code into Velocity macros to make them usable into Velocity scripts.  The macros will allow to display list of users and lists of groups as they are already displayed by `displayer_users.vm` and `displayer_groups.vm`.
Checking the class name is a bit too restrictive. You could check:

    #if ($arg.iterator())
      ## The argument can be iterated (the next 'foreach' should work).

Otherwise it's very good.