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
Merge pull request #891 from alphagov/56769702-first_published_at_bug

Fix editions with broken first_published_at

https://www.pivotaltracker.com/story/show/56769702