Dismiss stale approvals if invalidate_on_push: true

[AdamDeHovitz]

One feature natively support by github pull-request reviews is dismissing stale approvals:

I believe the invalidate_on_push option should similarly dismiss stale approvals to help clear the UI and also enable the rerequesting of reviewers by policy-bot's request_reviewers functionality.

[KnisterPeter]

@bluekeyes Not sure whom to tag here, so I've choosen you. :smiley: Are you up for a contribution here?

[KnisterPeter]

Also I think this is kind of related: https://github.com/palantir/policy-bot/commit/5e81dd9f65ca341104d917482739c1e38e2b0ba6 The main issue we currently have with this is:

• We have a PR
• A few approvals are done through github reviews
• Policy-Bot is configured to dismiss stale reviews
• A new commit is pushed

Then the github approvals are still there and not invalidated/dismissed. Using the github option to dismiss all of them is possible, but in our case a few teams would like to keep their existing approvals and a few won't. This isn't possible with github, but policy-bot could handle this case.

[aw185176]

Also hitting the same snag point onboarding engineers who are used to the GH UI being the up-to-date source of truth for state of a PR, if pointed in the right direction I may have cycles to put together a patch.