A `url` parameter is added to each old markdown file, so their url stay
`posts/...` and `essays/...`, but they're treated as the new `article`
page by Hugo.

The `url` is kept for Webmentions and other external resources, where a
simple rediret or alias won't work.

Also, Page Bundles are removed. Now only bare markdown files are
present.

- export old Discussion data as JSON and render them if found
- move legacy "Static Comments" to giscus.json
- remove giscus code