Implemented Symfony reverse proxy support for user context hash.

> Ref #26
> Relates to https://github.com/ezsystems/ezpublish-kernel/pull/1025

Finally the PR :smile: 
This adds support for Symfony RP for user context hash feature.

For this to work, you need to:
* Make your `AppCache` class extend `FOS\HttpCacheBundle\HttpCache` instead of `Symfony\Bundle\FrameworkBundle\HttpCache\HttpCache`.
* Ensure `USER_HASH_ACCEPT_HEADER`, `USER_HASH_HEADER` and `USER_HASH_METHOD` constants match your configuration (cannot rely on semantic config here as we're way too early for that).
* Ensure `SESSION_NAME_PREFIX` matches your environment.

In any case, it is possible to redefine those constants to fit your configuration.

## TODO
* [x] Implement user context hash support for Symfony RP
* [x] Clear documentation on how to set it up
* [x] Make user context hash response vary on sessionId(s) only as Cookie may contain undesired info (e.g. analytics)
* [x] Try to do some tests
does this not mean that there will be cookies in the cookie bag that could be used for hash calculation that then would end up in a mixed cache? why would we need to leave anything else than the session in the bag if its the only thing that matters for the hash?