HG-307 Use MediaWiki mechanism to get the Wiki title

Mercury API was returning WF value as a site name. It turns out that some wikias are using `'pagetitle-view-mainpage'` message to set the site name. 

With the changes below we use the same approach to get a site name as Wikia/app. 
Done. It's still quite inconsistent, though. The WikiFactory variable's name is `wgSitename` whereas we pass to Mercury `siteName` :blossom: