Attempt to tidy up organisations interface on editions

1. We've reduced the number of associations to just edition_organisations and replaced the old associations with methods that filter that.
2. Don't accept nested attributes for this anymore.
3. We've added lead_organisation and supporting_organisation setters that attempt to reuse edition organisation instances properly and avoid any duplicate keys errors we were getting with the old mechanism.

It's still more complex than a standard set of associations, but it's simpler than it was before.
Tidy up importer row instances to use new edition organisations API
Make controllers/views/features use new editions organisations API