resourceloader: Create ResourceLoaderContext::msg() method

resourceloader: Create ResourceLoaderContext::msg() method

This makes it easier to fetch messages without having to manually
call inLanguage() on each wfMessage() call, which is currently
causing some code forget this and use plain wfMessage() which
defaults to MediaWiki user language from session.

I've considered setting $wgLang or RequestContext::getMain(),
but that's still bad since modules get passed a context in their
methods and shouldn't be using global context either.

The warnings provided by MW_NO_SESSION are exactly what we want.

Change-Id: I1288fa5622d9f82d21bb66c8eb6518b90e7cddb4