Consider removing GitHub release + tags when merging a "revert release PR"

[mcmire]

If an engineer needs to redo a release due to to a bad changelog or incorrect version bump, then currently we ask them to submit a PR that reverts the changes in the release PR, then we ask them to recreate the release PR with the revised changes.

Unfortunately recreating the release PR will not work unless the tags and GitHub release that were created when it was merged are deleted first, and this step often gets forgotten. We should automate this step by detecting "revert release PRs" and automatically doing the removal.