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. 
You return `siteName` and the function is called `getSitename`. How about unifying it? I'd vote for camelCase.