replace slippers with django-cotton

main reason: slippers cannot pass request via context inside its
components, making it annoying to use template takes that take request.
more reasons: not actively worked on, no named slots, having to define
components in components.yaml + new components do not get registered
without restarting server

games/templates/cotton/button_start.html

@@ -0,0 +1,18 @@
+{% comment %}
+title
+text
+{% endcomment %}
+<button type="button"
+        title="{{ title }}"
+        autofocus
+        class="truncate max-w-xs sm:max-w-md lg:max-w-lg py-1 px-2 bg-green-600 hover:bg-green-700 focus:ring-green-500 focus:ring-offset-blue-200 text-white transition ease-in duration-200 text-center text-base font-semibold shadow-md focus:outline-none focus:ring-2 focus:ring-offset-2 rounded-lg">
+    <svg xmlns="http://www.w3.org/2000/svg"
+         fill="none"
+         viewBox="0 0 24 24"
+         stroke-width="1.5"
+         stroke="currentColor"
+         class="self-center w-6 h-6 inline">
+        <path stroke-linecap="round" stroke-linejoin="round" d="M5.25 5.653c0-.856.917-1.398 1.667-.986l11.54 6.348a1.125 1.125 0 010 1.971l-11.54 6.347a1.125 1.125 0 01-1.667-.985V5.653z" />
+    </svg>
+    {{ text }}
+</button>