Replace associations with scopes

Too many associations where leveraging scopes will do - this gets of
half a dozen.

I've also renamed the unpublished_edition method as we now have the
concept on an unpublished edition, which isn't quite the same thing.
Epic search/replace: archived becomes superseded

This frees us up to repurpose "archived" to mean documents that are
still visible on the site, but are considered to be out of date. It
also migrates the data for existing archived editions. I've done this
as a normal migration as want to minimise the time between the data
being migrated and the new code becoming active. This is because any
attempts to publish editions between the data being migrated and the
new code becoming active will likely cause an exception to be thrown.