Add new grammar forms for language names in Russian

CLDR provides translated language names. They are useful for showing
names by themselves in menus and lists, but it's often problematic to add them
to Russian sentences, because they need to be declined, so a message like
"This page is not available in the $1 language" is hard to localize.

This patch adds new cases for Russian -
"languagegen", "languageprep" and "languageadverb".
(The last one, as its name says, it's not actually a grammatical case,
but a transformation to an adverbial expression.)
This covers most of the needs for language names that MediaWiki supports.

Change-Id: Ib6a0afa5c3736f8b9b2e121cd752c53ee50fad75