Revamp GitHub review bot

[matthiasrohmer]

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 GoogleChrome/developer.chrome.com#5354 (comment) for recommendations).

[matthiasrohmer]

One linting rule that would be good to have is to check for linking to failing hrefs. Also see GoogleChrome/developer.chrome.com#5496.

[mrienstra]

Former review bot is GoogleChromeLabs/chrome-devrel-review-bot, yeah? Must be, just ran across GoogleChromeLabs/chrome-devrel-review-bot#20

In terms of linting / formatting Markdown files, some options taken from oxsecurity/megalinter docs/descriptors/markdown.md:

• DavidAnson/markdownlint
  • GH Action: DavidAnson/markdownlint-cli2-action
  • CLIs: igorshubovych/markdownlint-cli, DavidAnson/markdownlint-cli2
  • VS Code Extension: DavidAnson/vscode-markdownlint
• remarkjs/remark-lint
  • Can be extended to support different flavors of Markdown
  • Various integrations
  • GH Action starting point
• tcort/markdown-link-check
• nvuillam/markdown-table-formatter

PS: one of my motivations for suggesting linting / formatting Markdown: I had to "Save without Formatting" in VS Code to avoid making lots of spurious formatting changes when working on GoogleChrome/developer.chrome.com#5354. Surely I could turn off formatting for that repo somehow in VS Code, but it would be so much nicer to just have an official linter / formatter, so contributors don't have to examine several files to try to work out how things ought to be formatted before opening a PR. I didn't have to do that, but I've done it many times on other repos with inconsistent formatting, in an attempt to streamline the PR review process.