Add required require to prevent development 500
Set bulk_published flag when bulk publishing documents

This can be branched on further down the stack in order to show or hide fields
(updated_at) when a document has been bulk published.

I've added a spec which makes sure that the flag is cleared on subsequent
(manual) publication so updated_at will display.
Merge pull request #341 from alphagov/hide-updated-at-on-bulk-publish

Set bulk_published flag when bulk publishing documents