BAC-1231: change the way Cache-Control header is set

Remove ``X-Pass-Expires`` and rely on Cache-Control only

**Please don't merge it yet**

@Wikia/platform-team / @prein / @michalroszka 

This pull request introduces the following changes:

* ``WikiaResponse::setCacheValidity`` method has been simplified. It now requires just a single argument (cache TTL for both Varnish and the client). The second one is optional and is used when you want to have different TTL for Varnish (first argument) and the client (second argument).
* ``Expires`` response header is removed by Varnish. It's now removed from the code as well.