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`.
Again, checking the class name is too restrictive. You chould do:

    #set ($userReference = $user.extractReference('DOCUMENT'))
    #if (!$userReference)
      #set ($userReference = $services.model.resolveDocument($user))
    #end

Otherwise it's very good. But try to use came case everywhere, for consistency.