NixOS integration changes

[ntninja]

Each commit is basically self-explanatory:

• First commit is just about not logging secrets
• Second commit allows reading from a separate file
  • This is needed on NixOS since the Radicale configuration file is managed by the NixOS derivation, but we don’t have a the secret at that point yet (and also putting it in the derivation makes it world-readable in the Nix store)
• Third commit allows connecting to Modoboa using the Gunicorn Unix domain socket, improving the Radicale sandbox and making the internal connection more reliable
  • Unfortunately requests-unixsocket has a bug in not handling authentication data in the URL correctly (Authentication data not handled msabramo/requests-unixsocket#88), so the authentication data is instead passed using the auth= keyword (which is also the recommended way by requests documentation)