Failing test that exposes the bug.
Add migration to fix nil order values on existing editions
Use scoped ordering.

This removes the need to use the helper function which was breaking parts.

NOTE: IF we want the behaviour of new guides to have at least one part, that is something we should enforce in the models with a before_create. Doesn't belong in the view.
Fix tests and use specific selectors
Fix auto slug generation for newly added parts

Since moving to use nested forms, we now need to listen for a nested field added event and attach the auto slug generator handler accordingly.
Remove helper of a guide having at least one part

This enforcement, needs to be done on a model level, not within a view helper.
Betterer ruby
Use scoped ordering for programmes
Populate order field on newly added parts.
Merge pull request #112 from alphagov/fix_broken_parts

Fix broken parts