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 is performed or remove the published at when a document is withdrawn.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
I believe it was clarified with Holly that this field wouldn't be editable. Trello should probably have bene updated if true. Not sure what we're doing about imported ones though, good question.