Fix type of conversation ID in conversations API (#8961)

app/serializers/rest/conversation_serializer.rb

@@ -4,4 +4,8 @@ class REST::ConversationSerializer < ActiveModel::Serializer
   attribute :id
   has_many :participant_accounts, key: :accounts, serializer: REST::AccountSerializer
   has_one :last_status, serializer: REST::StatusSerializer
+
+  def id
+    object.id.to_s
+  end
 end