Clearer change notes for admin

• Update copy and styling, and reorder elements re new design
• Add placeholder text

https://www.pivotaltracker.com/story/show/56857726
Adds JS layer to edition change notes fieldset
Merge pull request #1003 from alphagov/clearer-admin-change-notes

Clearer change notes for admin

https://www.pivotaltracker.com/story/show/56857726