Preview links for pages cannot be shared

[ryelle]

At some point in the past, we were able to share preview links to other (logged in?) users to preview pages before they were updated. This stopped working (or maybe never really worked), but it would still be a useful feature to be able to share publicly a page as it's being edited.

Maybe this requires the public previews plugin? Or could be an update to the preview code from #108.

Ideally, I think this should allow non-logged-in users to view pages too, so that other teams (e.g., accessibility) can add feedback to pages before they're officially launched.