Add converted namespace names as aliases to avoid confusion.

Currently if the site language is zh and a user is using variant zh-tw,
namespace names from zh-hant are displayed because of the language
converter, but they're not accepted by MediaWiki as valid namespace names
by default because zh falls back to zh-hans.

For core namespaces, all converted namespace names are manually added as
$namespaceAliases in MessagesZh.php but it's not always done in extensions.
With this patch converted namespace names are automatically added as
namespace aliases when namespace aliases are loaded.

In some followup commit it makes sense to remove existing core namespace
aliases which were created for this reason.

Change-Id: I01873d9c64a9943afbb655d6203cec9ebd39fb72
Merge "Add converted namespace names as aliases to avoid confusion."