Add configuration for logout handler

Currently, setting `user_context: enabled` will throw an exception if no proxy client is configured because of the logout handler, which requires the client. This PR solves that by adding a `user_context.logout_handler.enabled` config option, which defaults to auto, so won't be available without a proxy client.
i propose to do $container->removeDefinition in the else case, rather than making the service abstract and un-abstracting it.

apart from that this would be good to merge. and helpful for some guy who is using the context with HttpCache and asked me on twitter yesterday what he needs to configure.