adds commands to promote/demote players

a few notes here:
We have five ranks: GUEST, REGISTERED, MODERATOR, ADMIN, OWNER.
Each rank can only promote from and to a lower rank then themselves. MODERATORs can only promote to REGISTERED, ADMINs can promote new MODERATORs... only the OWNER can demote (by promoting to GUEST). This is to prevent that an ADMIN can demote other admins and such shenanigans.