Private URLs for sharing drafts

Sharing a draft would be good to have. I've needed this myself and had to bodge it.

Every draft would need a corresponding temporary URL that would become unavailable after the post is published.

This is, of course, just a file, so the name would be random, but there needs to be a way to link together drafts and temporary URLs so we can delete them on publish, lest the content get stale after edits to published posts. Plus it's good housekeeping.