Migrate to JSON i18n

Procedure per https://www.mediawiki.org/wiki/Manual:GenerateJsonI18n.php
without shim. According to
https://www.mediawiki.org/wiki/Extension:BetaFeatures this feature
required MediaWiki 1.23, so there is no need for the shim.

Change-Id: I7cb283924924f63b92822c193821bc051bb94e0d