Fix editions with broken first_published_at

There are a dozen or so editions with a `first_published_at` timestamp that is broken (e.g. 0001-01-01). This fixes them, using the edition's `public_timestamp` if that is a sensible value, otherwise falling back to the latest edition's `first_published_at`

Tracker: https://www.pivotaltracker.com/story/show/56769702