Change notes for manual sections

Depends on https://github.com/alphagov/govuk_content_models/pull/175

https://trello.com/c/Gt022T8B/102-manuals-change-notes-on-manuals-for-publishers-3
I'm not sure we need to keep a `minor_update` field for this. If it's a minor update, then we don't allow editors to provide a change note. If it's a major update, we force them to provide one. So this could just be `change_note.present?`