Add published_at date to DSU on publish!

Trello ticket: https://trello.com/c/ZW0i6nya/362-add-first-published-date-to-the-dsu-finder-3-1

Add published_at date to drug safety updates on #publish! As this is part of the metadata for DSU we should store this in the DrugSafetyUpdate class not the SpecialistDocument class.
Make sure published dates are reset on withdrawal