Cleanup Special:PageTranslation more

Change-Id: Ia5655ae5fab8b885be3730e31ac6de7fe114220d
Create /source-language unit pages for translatable pages

* And even more simplifying to Special:PageTranslation
* Add TranslatablePage::getSourceLanguageCode
** To anticipate future features
** Not used everywhere yet

Change-Id: Ia263ee22639ab30506489df362c8de440eb8c077