nixos/keycloak: frontendUrl always needs to be suffixed with /

In some places, Keycloak expects the frontendUrl to end with `/`, so
let's make sure it always does.

nixos/modules/services/web-apps/keycloak.nix

@@ -54,6 +54,7 @@ in
 
     frontendUrl = lib.mkOption {
       type = lib.types.str;
+      apply = x: if lib.hasSuffix "/" x then x else x + "/";
       example = "keycloak.example.com/auth";
       description = ''
         The public URL used as base for all frontend requests. Should