Migrate to JSON i18n

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

Change-Id: Ifc1744e5e93fa9621e7e5f3b60ce58857e6ee987
Merge pull request #248 from SemanticMediaWiki/review/siebrand/json-i18n

Migrate to JSON i18n