Allow details and summary tags in HTML

Those two tags can be used to add expandable content, for example, to an exercise description.

config/application.rb

@@ -64,7 +64,7 @@ module CodeOcean
     end
 
     # Allow tables in addition to existing default tags
-    config.action_view.sanitized_allowed_tags = ActionView::Base.sanitized_allowed_tags + %w[table thead tbody tfoot td tr]
+    config.action_view.sanitized_allowed_tags = ActionView::Base.sanitized_allowed_tags + %w[table thead tbody tfoot td tr details summary]
 
     # Extract Sentry-related parameters from WebSocket connection
     config.middleware.insert_before 0, Middleware::WebSocketSentryHeaders