Being able to see old alerts on Sheriff-o-Matic has been requested before.

But also, I think it would be helpful for things like diagnosing flaky failures to have something like a history of an alert's past. ie: if the alert has been flickering on and off or if the alert was going for > 1 day, disappeared for hour, then came back. 

We could also use this to come up with a better model for deleting old annotations. ie: if an alert reappears after being gone for > 3 days, then we could delete annotations associated with it. 

This, I think, might make sense as part of a larger change in making alerts more meaningful, such as: https://bugs.chromium.org/p/chromium/issues/detail?id=681903 

Design doc pending, assuming the general idea of changing the alert schema sounds good.