Cleanup blank consultation outcomes

In today's view of the world, we want consultation outcomes and public
feedback to always have some summary text. But because we have legacy
responses that might not have a summary, we want to losen the
validation a bit so that they remain valid. To this end, we only
validate the summary is present if there are no attachments present.

https://www.pivotaltracker.com/story/show/54897632
Add data migration to clean up blank outcomes

This is legacy data, created as a result of the way nested attributes
used to be used to add/edit consultation responses.
Merge pull request #747 from alphagov/54897632-treat-consultations-with-blank-outcomes-as-not-concluded

Treat consultations with blank outcomes as not concluded

https://www.pivotaltracker.com/story/show/54897632