WikiaResponse: PLATFORM-57 related changes

* introduce three standard caching times
* introduce WikiaResponse::setCachePolicy method
* update unit tests
ArticleComments: use WikiaResponse::setCachePolicy method
AssetsManagerController: use WikiaResponse::CACHE_LONG
Use WikiaResponse::CACHE_STANDARD

Instead of hardcoded 86400 seconds
Merge pull request #3015 from Wikia/platform-57-standard-ttls

Configure cache TTL in a consistent, managable way