Auto-resolve FAILED slack notifications

In the new Slack integration, confirming a run updates the previously posted UNCONFIRMED notification to CONFIRMED. This is very good for Slack ops efficiency - on-call can scan the channel for today and see outstanding actions.

I'd like to request the same for FAILED. If a subsequent run from the same commit resolves the stack into FINISHED, the associated Slack notification should also resolve into an OK/green state. This would help with transient errors: retrying a run, or invalid operator decisions - an operator rejected a run flagged by policies, but the run should actually have been approved.