Make the Unpublishing factory more accurate

The edition associated with an unpublishing will have been put back to "draft"
from a "published" state, so needs to have publishing timestamps.
Update unpublishing schema

Neither the explanation or alternative_url are required fields - both can be
blank.
WIP - Unpublishing presenter accepts an unpublishing

This modifies the Publishing API presenter for Unpublishing to accept an
Unpublishing instance, which should simplify a bunch of code. It's been modified
such that it accepts both unpublishings that should result in a "unpubishing"
content item, and those that should generate a redirect.
Refactor up Unpublishing model

In the context of an Unpublishing, it makes less sense to treate deleted
Editions as a special case - better to just find it unscoped so we can get at it
regardless of whether it's been deleted or not.
WIP new Unpublishing presenters

Spiking out the rest of it.