Display published_at as a metadata field on a finder

DO NOT MERGE: Dependant on merging of:
https://github.com/alphagov/rummager/pull/301 and
https://github.com/alphagov/finder-api/pull/68
for index generation and facet generation

This pull request allows published_at metadata for Drug Safety Update documents to be viewed and searched by on finder-frontend.

Trello ticket: https://trello.com/c/ZW0i6nya/362-add-first-published-date-to-the-dsu-finder-3-1