Monitor: add history logs, either to emails or in dashboard.

Site owners receive Monitor emails, and some delete them. This isn't a very convenient way to share the "big picture" of what could be consistent server downtime or an overall failing server.

If we could find a way to log downtime reports for a particular site for just a 24-hour period, and include those historical downtime dates/times from the 24-hour period in the Monitor email, we will not only make the "big picture" easier to digest, we'll also provide something that is far more useful for the hosting provider to investigate.

Note that I'm not advocating the creation of a developer-resource-intensive UI or Dashboard for the Monitor itself, I'm just advocating 24-hour logging and reflection of that log in the Monitor emails.

I consider this issue somewhat of companion to #2796 but a bit lower priority, hence the separate issue.

Start by reviewing how Monitor emails are generated and compare the proposal with companion issue #2796. Clarify the 24-hour downtime history behavior and email presentation; done means historical downtime dates and times are included for the relevant site without requiring a new dashboard.

Written by the indexing model from the issue text.