feat(deertopia): Foundational Authelia setup

By 'foundational,' I mean that a demo is working correctly.  Work will
continue in a follow-up commit integrating existing services with LDAP
and Authelia. ♥

modules/nixos/deertopia/nginx.nix

@@ -38,7 +38,7 @@ in
           };
           directory = lib.mkOption {
             description = "Host's root directory.";
-            type = lib.types.str;
+            type = lib.types.nullOr lib.types.path;
             default = "${cfg.root}/${name}";
           };
           user = lib.mkOption {