Layout fixes on interactive article pages, reinstate margins.

Fixes layout at smaller breakpoints (bugs resulting from global breakpoint changes).

This PR also reinstates the left and right grey margins, after consultation with @Pearson82 and interactive team. Emailed wider interactive team notifying them of change, will await their okay before submitting.

Before: 
![1a](https://cloud.githubusercontent.com/assets/813383/4359454/f4694356-4271-11e4-8dea-bcb6f5925f9a.png)
![2a](https://cloud.githubusercontent.com/assets/813383/4359456/f7b59262-4271-11e4-9716-0e908ac3369d.png)

After:
![1b](https://cloud.githubusercontent.com/assets/813383/4359460/fc4290a0-4271-11e4-8cf4-33a5288c416e.png)
![2b](https://cloud.githubusercontent.com/assets/813383/4359463/005a126c-4272-11e4-9e21-57b3d5b29f50.png)