Fix first published at label for import documents

Imported documents need a first published at date to be set. Where as
new or edited documents need exectly the inverse.

https://www.pivotaltracker.com/story/show/45484805
Merge pull request #314 from alphagov/speed-first-published

Fix first published at label for import documents

https://www.pivotaltracker.com/story/show/45484805