Revamp GitHub review bot

The former review bot based on a Glitch backend is currently broken, probably due to auth issues. Before fixing it, it might be good to outweigh wether moving to a GitHub action that can be shared for both web.dev and developer.chrome.com might be the better approach.

When doing this we should also re-evaluate the current set of rules, think about worthwhile additions (like enforcing thumbnail images for blog posts) and probably implement spell-checking (see https://github.com/GoogleChrome/developer.chrome.com/pull/5354#issue-1590144636 for recommendations).