Add GitHub PR previews

[bep]

As in a green or red check with a link to preview on https://some-pr-id.example.org.

There are several interesting challenges in the above, but I think all of them doable:

• We don't want to duplicate Amazon S3 buckets for simple spelling corrections, so we could
  • Build the Hugo site twice and get a diff
  • Create a bucket with only the changed files.
  • Add a union file system on top of S3
• Certificates handling with dynamic sub domains