Migrate to JSON i18n

Procedure per https://www.mediawiki.org/wiki/Manual:GenerateJsonI18n.php
with shim.

Change-Id: Iae464affd3fee133c51bef4468ae1f44925632c6
Reviewed-on: https://gerrit.wikimedia.org/r/122309
Reviewed-by: Raimond Spekking <raimond.spekking@gmail.com>
Tested-by: Siebrand <siebrand@kitano.nl>