Add a 'namemsg' parameter to $wgExtensionCredits for localizable names

Works similarly to 'description' and 'descriptionmsg', but 'name'
is still required because of weird shenanigans Special:Version does.
(And would be a good idea anyway for backwards-compatibility with
older MediaWikis.)

The primary use-case is skins (which have already traditionally had
translateable names in MediaWiki, but weren't always shown on
Special:Version), but there's no reason why regular extensions can't
use this too.

Skins which already have a translated name for Special:Preferences
('skinname-<skin>' messages) can reuse the same message here.

Change-Id: Iae6f770a8fe1968670429c22aefc1ae55e8dba6f
Merge "Add a 'namemsg' parameter to $wgExtensionCredits for localizable names"