Add ability to notify when an Alert is cleared

[timgaunt]

Leading on from #1126 it would be really helpful to know when an alert has gone from an "In Alert" state to "Alert Cleared" state so people can stand down (or no panic) if by the time they see the Slack notifications it's cleared.

Here's an example of an Alert that came in overnight and then was re-triggered the following day (but was out of alert state 0000-0925):

I think re-using the "In Alert" notification concept would work but I think we would want a separate configuration in case we wanted custom messaging etc.

Here's an example of how it looks with DataDog for comparison:

[nblumhardt]

Thanks for the suggestion. This is definitely aligned with where we'd like to go with Alerts; timing is uncertain but it's good to have on the radar 👍