Publications can be imported without a publication date

They can't be set to draft without it being filled in though.
Importing NewsArticles with a blank first published is allowed

This means that news articles validate the presence of first_published_at, but only when they're not imported.
Speeches can be imported without delivered on date

If delivered_on is blank, we also throw away any delivered_by information in the file - because we want to make sure we find the correct role for the deliverer based on the date of the speech.  If we don't have the date we can't be sure we have the correct role, so it's better to force the importing user to fix this during speed-tagging.
Imported Consultations do not need to specify opening or closing dates
Merge pull request #182 from alphagov/import-without-dates

Import without dates