Correctly display preview of posts being edited.

This commit changes the color of the preview if the post you are previewing is still being edited.
![preview-edit](https://cloud.githubusercontent.com/assets/3243212/5118209/1546e40a-705f-11e4-8448-33025268f0cd.png)
I changed div to article because it seemed like the correct way to do it, and I don't think it causes any problem.
But I don't really know much about it so I can change it if it's bad.