Allow serializing local-only toots in backup service

Fixes #1153

app/serializers/activitypub/note_serializer.rb

@@ -25,7 +25,7 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
   attribute :closed, if: :poll_and_expired?
 
   def id
-    raise Mastodon::NotPermittedError, 'Local-only statuses should not be serialized' if object.local_only?
+    raise Mastodon::NotPermittedError, 'Local-only statuses should not be serialized' if object.local_only? && !instance_options[:allow_local_only]
     ActivityPub::TagManager.instance.uri_for(object)
   end
 

app/services/backup_service.rb

@@ -149,7 +149,8 @@ class BackupService < BaseService
     ActiveModelSerializers::SerializableResource.new(
       object,
       serializer: serializer,
-      adapter: ActivityPub::Adapter
+      adapter: ActivityPub::Adapter,
+      allow_local_only: true,
     ).as_json
   end