Add unread indicator to conversations (#9009)

app/serializers/rest/conversation_serializer.rb

@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
 class REST::ConversationSerializer < ActiveModel::Serializer
-  attribute :id
+  attributes :id, :unread
+
   has_many :participant_accounts, key: :accounts, serializer: REST::AccountSerializer
   has_one :last_status, serializer: REST::StatusSerializer