Add exclusive lists (#22048)

Co-authored-by: Liam Cooke <liam@liamcooke.com>
Co-authored-by: John Holdun <john@johnholdun.com>
Co-authored-by: Effy Elden <effy@effy.space>
Co-authored-by: Lina Reyne <git@lina.pizza>
Co-authored-by: Lina <20880695+necropolina@users.noreply.github.com>
Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/serializers/rest/list_serializer.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class REST::ListSerializer < ActiveModel::Serializer
-  attributes :id, :title, :replies_policy
+  attributes :id, :title, :replies_policy, :exclusive
 
   def id
     object.id.to_s