Add a published_at field to RenderedSpecialistDocument

To avoid updated_at being modified every time we create a new draft, we
want a new 'published_at' field which is set only upon publish.

- [x] Merge content-models pull request https://github.com/alphagov/govuk_content_models/pull/204
- [ ] Update version of content models used
- [ ] Fix republishing script to operate on all types of specialist document
This will set the `published_at` time to be the time we do the export to Content API, rather than the time the latest version of the document was originally published. Ie it means all the timestamps will be set to the current time when we republish everything.