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 is performed or remove the published at when a document is withdrawn.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've just checked with Holly and this is the desired behaviour. I'll update Trello.