OasisController: introduce addSkinAssetGroup method

This change introduces ``OasisController::addSkinAssetGroup`` helper method for adding AssetsManager groups to the main Oasis JS request.

An example:

```
/__am/123456/groups/-\/oasis_shared_core_js,adengine2_js,oasis_shared_js,oasis_user_js,liftium_ads_js,toc_js,qualtrics_zone_code_injector_js,qualaroo_js,wall_notifications_js
```

Calling this method is much simpler than binding to ``OasisSkinAssetGroups`` hook that usually requires copying the logic (see the WallNotifications code that is ported to this helper method as an example).

Replacing ``$this->response->addAsset`` will remove one additional HTTP request by requesting all needed AssetsManager groups in one single request.

@Wikia/platform-team / @sebastian-wikia  / @nandy-andy / @RafLeszczynski