Allow Hackbot UI users to opt-in to receiving an email when the job has completed

These days, if I invoke Hackbot via the UI, I keep the tabs open in a group somewhere, and will periodically check in on them to see how they're doing. What'd be lovely is if I could just close the tab, and receive an email (and/or other notification) when the job completes so that I can then re-open the job to inspect what occurred.