OasisController: introduce addSkinAssetGroup method
WallNotifications: use OasisController::addSkinAssetGroup

And save one precious HTTP request :)
Wall: update unit tests
RelatedPage: use OasisController::addSkinAssetGroup
Revert "RelatedPage: use OasisController::addSkinAssetGroup"

This reverts commit 5383926fa1df25f66a14f498fc51b1582cb91b2e.
RelatedPages: keep it compatible with Monobook
Merge pull request #3192 from Wikia/addSkinAssetGroup

OasisController: introduce addSkinAssetGroup method