Normalise Artefacts need_ids

The "1" was a fudge when need_ids were required but business content did not
have need_ids. By making them blank, we can then make these records
editable-if-blank.
Make blank need_ids editable

We should be very careful about allowing changes to need_ids, but blank ones
should be settable.
Merge pull request #76 from alphagov/make_need_id_editable_when_blank

Make need id editable when blank