Main dispatcher, the entrance point for all 'Manage Membergroup' actions.
It forwards to a function based on the given subaction, default being subaction 'index', or, without manage_membergroup
permissions, then 'settings'.
Called by ?action=admin;area=membergroups.
Requires the manage_membergroups or the admin_forum permission.
uses ManageMembergroups template.
uses ManageMembers language file.