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
So it won't actually resolve the bug at all. Nice. Well done me.