Automatically update deploy failure alerts when mitigated

[IanWoodard]

The deploy failure alerts are incredible useful, but it would be nice for there to be way of knowing if the problem is still ongoing. This could mean replying to the thread when the pipeline is unpaused (assuming it was paused) or providing engineers with a means of marking the problem as resolved. The latter would also enable us to have engineers let us know if an alert was a flake which could prove useful, but this is out of scope for now.