"Updated" not "Updated at" for date metadata

The footer area (and other parts of GOV.UK) refer to dates as "Updated:" and
"Published:", not "Updated at" etc.

We should follow this convention.
Only display one of updated or published date to the user.

The document history becomes the source of when the document was first published.
Override displayed dates for DSU's
Merge pull request #74 from alphagov/normalize-published-and-updated-dates

Normalize published and updated dates