Admin tool for collections

Last week we finished work on front-end work for collections (https://github.com/Wikia/app/pull/768). The data for this front-end part was pulled from WikiFactory variable. 

This time we're ready to release admin tool for this feature. Thanks to this changes (mostly in models and SpecialManageWikiaHome extension) our Marketing Team will heave easy way to manage collections (add/remove them, add/remove a wiki to/from a collection). Also, since we were changing Special:ManageWikiaHome we added possibility to mark a wiki as "Official".

@lgarczewski @mroszka @Grunny do i18n/l10n review, please. There are changes only in one .18n.php file:
https://github.com/Wikia/app/pull/851/files#L2R113