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
Add keyword filtering to the topics index
Remove debug output from test
Rename Edition::AuditTrail#last_audit_trail_version_event

I found this name confusing when reading the code. The method returns the
latest VersionAuditEntry where the `state` stored against the wrapped
PaperTrail Version object matches the state we're interested in.
Rename Edition::AuditTrail#audit_trail

I found it confusing that `Edition#audit_trail` returned an audit trail for
all editions belonging to the document. I think renaming the method to
`Edition#document_audit_trail` makes this more obvious.
Merge pull request #188 from alphagov/topic-filter

Add keyword filtering to the topics index
Merge pull request #182 from alphagov/import-without-dates

Import without dates