Add published-at to RenderedSpecialistDocument

[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)

We want to store published at separately from updated_at so we control when it's updated, not mongoid.