Show 'Published at' not 'Updated at' for the first edition of a doc

When a document hasn't been updated since publication, we want to show "Published at" in the metadata section, not "Updated at". This is more meaningful for a user.

[Trello ticket](https://trello.com/c/y0l09tYY/493-when-a-doc-is-first-published-show-text-as-published-when-it-is-published-again-show-text-as-updated)