HG-483: Add wikiCategories to WikiVariables response

As a part of the effort to unify tracking between MW and Mercury, we need to pass the value of `wgWikiCategories` to Mercury. I wanted to keep this DRY, and that's why the `WikiFactoryHubHooks::getWikiCategories` logic was extracted from `WikiFactoryHubHooks::onWikiaSkinTopScripts` :oden: 

Related to: https://github.com/Wikia/mercury/pull/464
/cc @kvas-damian @hakubo 
While this method is shared between ```WikiFactoryHubHooks``` and ```MercuryApi``` I'd suggest moving it to ```WikiFactoryHub``` class.