jupyter_scheduler to add delete [All] button for completed jobs

[yangoo9]

Hi

I have ran with jupyter_scheduler extension for auto scheduling my notebook and I will keep growing logs such as Completed jobs, that I have to manage to remove. But there has no Delete All button to remove all the completed logs, so I have to go through every logs to remove it manually.

I would request to add the delete all completed logs button in next version would be really appreciate. Also attached file for your reference. thanks