Normalize published and updated dates

* Reword "Published at" to "Published" and "Updated at" to "Updated" for consistency
* If a document is the first edition, display "Published", otherwise, display "Updated". In both top metadata and in the footer component
* Never show a "Published" date for Drug Safety Updates (this is the "issued_date" in the actual document metadata, which appears at the top of the document). Show an "Updated" date if the document isn't the first edition.

This depends on https://github.com/alphagov/static/pull/514 which must be deployed before this change.

[Trello ticket](https://trello.com/c/6uURnQ0n/496-footer-date-metadata-cleanup-published-updated-at-dsu-special-casing)