Display background tasks and cron tasks in the admin console

[tompollard]

Once we've updated the approach for dealing with background and scheduled tasks, we should consider adding a page to the admin console that displays them. e.g.

• what are the regular cron tasks that are scheduled to run?
• what tasks are currently queued?
• what tasks are actively running in the background?
• what tasks have recently run (e.g. past 100 tasks)