remove Vary: Cookie

Any access to request.user will cause the session middleware in django to to put a Vary: Cookie header. This header means the page is only cacheable for requests with the same cookies (in effect a private cache per user). We don't want to do this for pages that don't access the request.user. But we want to record the reversion of the currently logged in user if they did change something. So lets do the same thing the session middleware does and check if it was accessed.