Change note tweaks

I think these conditionals would be better done along the lines of:

```ruby
def change_note_required?
  previously_published? && !minor_update?
end

def change_note_provided?
  if change_note.present?
    result = true
  else
    …
  end

  result
end

def valid?
  (change_note_required? && change_note_provided?) && entity.valid?
end
```

At the moment the logic is a bit split out across different methods which makes it a bit awkward to follow.