VE-1145: Handle global/local favicon

https://wikia-inc.atlassian.net/browse/VE-1145 @inez 
Another method you create: isFavicon takes $title as a parameter. I would expect getFavicon to return title as well, or file, but not url. Should it be called getFaviconUrl (or getFaviconFullUrl - whichever fits MediaWiki "standard").