Enable per-wiki addition to 'translationadmin' group

This patch enables per-wiki configuration for
$wgAddGroups to the 'translationadmin' user group.

Currently, only bureucrats are able to add users to
that group, which is somewhat limiting for wikis
without any bureaucrats.

Bug: 58123
Change-Id: Id630b119ba50dd2214f4f8ca5a0ef3b7301c9db7