Invite admins to automatic backups failure topic.

https://meta.discourse.org/t/if-automatic-backup-fails-there-should-be-a-warning/64461

lib/backup_restore/backuper.rb

@@ -259,9 +259,15 @@ module BackupRestore
       log "Notifying '#{@user.username}' of the end of the backup..."
       status = @success ? :backup_succeeded : :backup_failed
 
-      SystemMessage.create_from_system_user(@user, status,
+      post = SystemMessage.create_from_system_user(@user, status,
         logs: Discourse::Utils.pretty_logs(@logs)
       )
+
+      if @user.id == Discourse::SYSTEM_USER_ID
+        post.topic.invite_group(@user, Group[:admins])
+      end
+
+      post
     end
 
     def clean_up