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/models/list.rb

@@ -10,6 +10,7 @@
 #  created_at     :datetime         not null
 #  updated_at     :datetime         not null
 #  replies_policy :integer          default("list"), not null
+#  exclusive      :boolean          default(FALSE)
 #
 
 class List < ApplicationRecord