Create GH Action that flags "octocat" commits

We generally try to avoid "octocat" commits, i.e. commits in PRs which are authored using an e-mail that's not associated to any GH user account. This makes it harder to attribute commits to users, e.g. they won't show up in the "Contributions" statistics.

Instead of manually asking users to rectify this situation, could we have a GH Action which does that automatically, similar to the formatting check which fails a build if formatting is off.