SpecialCentralAuth: Link global group names

Previously they were a comma-separated list that wasn't very
helpful.

Special:GlobalGroupPermissions makes for a good hub as it links
to everything else (name, members, rights).

Also fix incorrect call to htmlspecialchars on the result of
Language::commaList which already escaped the separators (it
was still supposed to escape the names, but that should've been
done on the array directly).

Change-Id: I24af208641a371c1019873c4d59b99c225551bf2
Merge "SpecialCentralAuth: Link global group names"