Add duration and timing of last downtime to `uptime` listings

Migrated from https://github.com/myplanetdigital/pantzbot/issues/35:

> When we get a pagerduty alert about downtime, often it was shortlived, so it's nice to be able to find out how long it lasted, right from the chat. If it only lasted for one ping (5 minutes), then it's likely minor, but if it lasted longer, then it's a sign that it's a bigger deal.
>
> cc: @timfernihough @yrassoulli