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 "รค").
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