Grant Edit Assignee and Add Patchset to refs/*

The default inherited policy for editing assignee and
amending patch sets, is to allow by patch owner and
project owner to do so (where project owner for mediawiki
is Gerrit Administrators). The mediawiki users group has
been vetted and should be allowed this capability so as
not to hinder their productivity to an unreasonable
degree.

• project.config[diff]