Accept BCP 47 codes as aliases for nonstandard variants

Accept BCP 47 codes as aliases for nonstandard variants

The browser Accept-Language header uses BCP 47 codes, which don't
precisely match our internal mediawiki variant names in a number of
places.  Allow proper BCP 47 codes to alias our internal variants
for: Accept-Language parsing, URL parsing, user preferences, and
explicit enumeration of codes in LanguageConverter rules.

Change-Id: I8468a56d5b88f5786abd0a17b67bda2f1687fd0c