Raise nginx upload size limit to 20M

Default client_max_body_size of 1M caused 413s on typical photo uploads.

module.nix

@@ -126,6 +126,7 @@ in {
             proxyPass     = "http://127.0.0.1:${toString cfg.port}";
             basicAuth     = "gallery";
             basicAuthFile = cfg.nginx.htpasswdFile;
+            extraConfig   = "client_max_body_size 20M;";
           };
         };
       };