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_group_sm.html

@@ -0,0 +1,6 @@
+<c-vars color="gray" />
+<div class="inline-flex rounded-md shadow-sm" role="group">
+    {% for button in buttons %}
+        <c-button-group-button-sm :href=button.href :text=button.text :color=button.color />
+    {% endfor %}
+</div>