privoxy: upstart to systemd conversion, actions file editing

PRIVOXY CURRENTLY DOES NOTHING.

If no actions or filter files are specified, which is what is does now, privoxy performs no actions or filtering.
This can be seen by enabling privoxy and loading the following page. 
http://config.privoxy.org/show-url-info?url=nixos.org

This commit copies the upstream actions and filters installed with privoxy to /var/lib/privoxy and includes them into the configuration file. This restores privoxy's expected behavior.

I also added an option to enable the configuration editor that is accessible to proxy users. This is why the configuration now lives in /var/lib/privoxy.

Managing the filtering configuration would be nice but becomes complicated fast, if someone had a static filter file they could always write it to the nix store and include it using 'extraConfig'.
Are you sure about ```chmod 0755 -R ${stateDir} ${cfg.logDir}```? I don't know privoxy, but I have to ask since this creates world readable files.