FIX: unable to create new categories

Previous attempt at 70adb940 missed the critical "everyone" group from
staff, leading to a case where staff was no longer able to create categories

app/serializers/application_serializer.rb

@@ -25,4 +25,12 @@ class ApplicationSerializer < ActiveModel::Serializer
   def cache_fragment(name)
     ApplicationSerializer.fragment_cache[name] ||= yield
   end
+
+  def cache_anon_fragment(name, &blk)
+    if scope.anonymous?
+      cache_fragment(name, &blk)
+    else
+      blk.call
+    end
+  end
 end

app/serializers/site_serializer.rb

@@ -50,7 +50,9 @@ class SiteSerializer < ApplicationSerializer
   end
 
   def groups
-    object.groups.pluck(:id, :name).map { |id, name| { id: id, name: name } }.as_json
+    cache_anon_fragment("group_names") do
+      object.groups.order(:name).pluck(:id, :name).map { |id, name| { id: id, name: name } }.as_json
+    end
   end
 
   def post_action_types