Push Unpublishings to Publishing API

Re-opening as new pull request  (original pull request: https://github.com/alphagov/whitehall/pull/1891)

This updates whitehall to push either an "unpublishing" content item or a redirect to Publishing API when a document is unpublished. It also includes a data migration to push all existing unpublishings to the Publishing API as "republish".

Stories:

- https://trello.com/c/BQodP4gq/58-handle-unpublished-splash-pages-in-the-new-publishing-2-0-stack
- https://trello.com/c/M9rXIG1D/59-handle-redirect-unpublishings-in-the-new-publishing-2-0-stack
is there a risk of triggering an email alert if we send a 'major' change rather than a 'republish'. Would it be preferable to invent an explicit verb `archive` to reflect the action of archiving a document? This would then allow things listening on the message queue to act appropriately.

Any thoughts @rboulton @alext ?