Previews are deleted when the main site is updated

[brian-rose]

Deploying to the main site (e.g. by merging a PR, or by manually running the publish-book workflow) currently causes ALL PR previews that were previously deployed to labeled subdirectories to be deleted.

Desired behavior:

• All PR previews should be independent of the main deployment.
• Deploying to the main site should delete pages that have been removed from the main branch of the source (PR previews have undesired side effect for main page (deleted html pages are persisting) #120). I think this is working now.
• But anything previously deployed to the /_preview subdirectory should be unaffected. Those should remain visible as long as the related PR is open.