mediawiki.api.watch: Use formatversion=2 for API requests

mediawiki.api.watch: Use formatversion=2 for API requests

With formatversion=2 the JSON response uses UTF-8 instead of escape sequences
with hex for encoding of non-ASCII characters (e.g. "\u00e4" for "ä").

formatversion=2 has a different syntax for boolean values:
  formatversion=1: "watched": ""
  formatversion=2: "watched": true

Consumers of this API that process the response with
  response.watched === ''
must change this to
  response.watched === true

Change-Id: I087ab538b9400d774f8c2c6a8c0f55a33fab6044