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/client/index.html

@@ -90,6 +90,13 @@
       document.documentElement.className = "preload";
     </script>
 
+    <!-- Cloudflare Turnstile -->
+    <script
+      src="https://challenges.cloudflare.com/turnstile/v0/api.js"
+      async
+      defer
+    ></script>
+
     <!-- Publift/Fuse ads -->
     <script
       async
@@ -201,7 +208,10 @@
       </div>
     </header>
     <div class="bg-image"></div>
-
+    <div
+      id="turnstile-container"
+      class="fixed top-1/2 left-1/2 -translate-x-1/2 -translate-y-1/2 z-[99999]"
+    ></div>
     <gutter-ads></gutter-ads>
 
     <!-- Main container with responsive padding -->