Trigger on reverting a PR so I can label the PR as "reverted"

[morrismatts]

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It doesn't seem it's possible to trigger an action when clicking the "Revert" button on a merged PR. I'd really like to label all reverted PRs as part of analyzing Change Failure Rate, but also just to make it easier to identify those PRs without having to read through the history of each PR manually.

I realize that the UI is not the only way to revert a PR, but it's the most common way in our workflow.