Hide updated_at if document was bulk published

Specialist-publisher can bulk publish documents. A piece of metadata --
"bulk_published" -- is set to true when this happens.

When a document is bulk published, we don't want to show "Updated at", as this
confuses users.

Whilst this relates to https://github.com/alphagov/specialist-publisher/pull/341 it does not depend on it and can be merged in isolation.