Add published at to DrugSafetyUpdates

This pull request adds functionality to drug safety updates in order to automatically set published_at when publish! is performed or remove the published at when a document is withdrawn.

Trello ticket: https://trello.com/c/ZW0i6nya/362-add-first-published-date-to-the-dsu-finder-3-1
The trello card implies that this needs to be editable -- or at least respect the value that comes from an import. Blindly choosing `Time.now` means we're not able to respect the `published_at` for historical documents as they'll be overwritten on publish.