PLA-1823 Leave Special:Promote for staff only

Leaves Special:Promote accessible to staff/helper only.
Other groups of users will receive information about this feature being disabled.
'promotedisabled' permission name is not obvious. It should be more like 'restricted_promote' or "promote_for_coterie_only" IMnshO

Also I wonder why permissions for normal 'promote'  couldn't just be changed ??