Don't leave empty cells for blank values in changesets:

* Show they were <blank> (grey out a little, italicise)
* if we're not using table headers for the changes, try
  and be explicit that this was a change by using a visual
  trick like
  field_name | old_value > new_value
Merge pull request #16 from alphagov/tweak-versions-ux

Don't leave empty cells for blank values in changesets: