Adapt to core api change hitobito/hitobito#998281907

For performance reason we no longer load all people to check if a person
is subscribed, so we have to cater for that here

app/domain/jubla/mailing_lists/subscribers.rb

@@ -8,6 +8,10 @@
 module Jubla::MailingLists::Subscribers
   extend ActiveSupport::Concern
 
+  def subscribed?(person)
+    super && excluded_by_contact_preference.where(id: person.id).empty?
+  end
+
   def people_as_configured
     super.where.not(id: excluded_by_contact_preference.select(:id))
   end