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 is performed or remove the published at when a document is withdrawn.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 ticket says `first published`. We need to speak to Holly about what we do when a document is withdrawn -- i.e. do we keep the original first published at or do we nullify it.