All Specialist Documents now have a published_at field.

This will be required to fulfil [this trello ticket](https://trello.com/c/K1rcmRCw/242-bug-fix-updated-at-dates-on-all-specialist-publisher-documents-so-that-they-don-t-change-when-a-draft-is-saved-2)

- [x] Merge content-models pull request https://github.com/alphagov/govuk_content_models/pull/204
- [ ] Update version of content models used in content_api
- [ ] Deploy new specialist-publisher and republish all existing documents, to give them a published_at field