Turnstile: require token before joining a multiplayer game (#2572)

When user tries to join either a public or private multiplayer game, the
turnstile callback is triggered, and the turnstile token is passed to
the server when joining a game.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/core/configuration/ProdConfig.ts

@@ -11,4 +11,7 @@ export const prodConfig = new (class extends DefaultServerConfig {
   jwtAudience(): string {
     return "openfront.io";
   }
+  turnstileSiteKey(): string {
+    return "0x4AAAAAACFLkaecN39lS8sk";
+  }
 })();