Add overview page for celery task results

Especially for scheduled tasks in order to inform the admin

added type::feature workflow::discussing labels

assigned to @debdolph

Maybe add to system status page

created merge request !281 (merged) to address this issue

mentioned in merge request !281 (merged)

added 2h of time spent at 2020-05-22

mentioned in commit 307532e9

closed via merge request !281 (merged)