Move change notes toggle to its own link

People didn't realise that you could click the date to see the document
history. This makes it very clear when there is a history that you can
click the toggle to see it.

https://www.pivotaltracker.com/story/show/49294633
Merge pull request #670 from alphagov/change-notes

https://www.pivotaltracker.com/story/show/49294633

Move change notes toggle to its own link