add path-prefix get param, and place it before language if present

@drsnyder 
changes on vignette side to account for new path-prefix get param. this will replace the lang get parameter (prefixes are not always languages), but leaving that get parameter in for now until the path-prefix code makes it to production on the mediawiki side and all parser caches expire.

See https://github.com/Wikia/app/pull/5130 for changes in PHP client