A bunch of MediaWiki 1.24 things to make Refreshed 1.24-compatible

* converted PHP-based i18n to JSON
* /skins/common/ no longer exists, so instead of bundling certain files which used to reside there into the skins.refreshed module, just load the mediawiki.skinning.interface module in SkinRefreshed::setupSkinUserCss()
* added class="mw-body" to div#content so that we can use the mediawiki.skinning.content.externallinks RL module for external link icons
* some random cleanup
* bumped version number to 2.2