Integrate Special:MyLanguage from the Translate extension

In practice, this means the MediaWiki canonical way to uniquely and
humanly name translation pages on a multilingual wiki becomes
$source_page_title + '/' + $target_language_code, irrespective of
the enabling of the navigational subtitle and of the relative
titles (..) syntax for linking and transcluding, which are unrelated
and keep being controlled by $wgNamespacesWithSubpages.

This syntax has been prevalent on most wikis including mediawiki.org
since at least 2009, also thanks to #titleparts in ParserFunctions.

The part of the special page's functionality that made links to it red
when destination page wasn't present will be integrated with a follow-up
commit because they require more performance work.

I'm also including some Title tests that were written for testing the
redlinking functionality but are useful in general.

Bug: 66762
Change-Id: I520077c931431b5919e0208f75c20b5b25f3159d
Merge "Integrate Special:MyLanguage from the Translate extension"