Data migration to fix failed archivings

There are a small handful of documents that have multiple "published"
editions. This data migration repairs things, archiving all but the
most recent "published" edition.

For a period, we had validations that meant archived editions were not
passing validation. This should have meant that a publishing attempt
would fail because the archiving of previous editions would blow up,
but because EditionWorkflowController rescues
ActiveRecord::RecordInvalid, we were publishing the new edition and
carrying on, despite the exception thrown during the archive attempt.
The validations have been corrected now, so we shouldn't see this
anymore, but we should still remove the rescue_from from the
controller once we have reliable service objects for all workflow
actions.
Merge pull request #988 from alphagov/fix-failed-archivings

Data migration to fix failed archivings

Part of https://www.pivotaltracker.com/story/show/57899408