Provide month names via mediawiki.language.months RL module

* Deliver appropriate messages via ResourceLoader, for nominative-case,
  genitive-case and abbreviated month names
* Define mw.language.months which provides access to the above data in
  a structured manner

This supersedes the old way to of accessing these using wgMonthNames
and wgMonthNamesShort mw.config variables.

Genitive month names were previously not available at all.

Bug: 46496
Change-Id: I9f48b860d1078a31d620f6f95dde1f19c1c31f79