Allow Commons admins self-adding translationadmin group

Per
https://commons.wikimedia.org/wiki/Commons:Requests_for_comment/Assign_translationadmin-right_to_admins
(100% support; > 1 month open; usual RfC procedure at Commons; not
announced as a watchlist notice)

Allowing Wikimedia Commons administrators adding oneself to the
translationadmin-user group.

Converting links per Odder's comment into "Bug <number>".

Bug: 54732
Change-Id: Ia539db2bcb2e1deee5997a4c7fe4cd9f742d43a8