Make HostSiteSelector working again

The behavior of the subrequest framework has changed in Symfony 2.3 (see https://github.com/symfony/symfony/issues/7551).

This PR make calls to `render` in SonataPage working again.

For subrequests, the original request pathInfo is automatically added as parameter (the Symfony `container` Twig helper is overrided) and handled in `HostSiteSelector`.