Statistical data set date manipulation

Make statistical data sets behave more like news articles:

* allow editing of first published at
* importing them required first published information

Also:

* migrate existing imported statistical data sets to have "Data set updated." as the change note of the first edition
* importing allows a change_note field to set the change note of the first edition (if blank it falls back to above).

This is because it's very unlikely that an imported data set was actually *first* published on that date, so we give it a non-specific change note to override the default change note displayed for blank initial change notes.