HG-228: Ads code should nor rely on global variables to work

This is still work in progress....

This PR separates the Ads loading and management logic in separate module: `[Wikia|Mercury].Modules.Ads`.

The class has to be set up before being used by calling `init` and passing the URL to the ads script on MW. 

PR in app https://github.com/Wikia/app/pull/5108