Multimedia side margin banding

The side margins on multimedia containers are not contrasted enough.

### Before
![google chrome](https://cloud.githubusercontent.com/assets/80089/3844113/52115016-1e46-11e4-849f-bb569a26985d.png)

### After
![google chrome](https://cloud.githubusercontent.com/assets/80089/3844116/59a9518e-1e46-11e4-92e6-e6bed5a2b36c.png)

I'd avoid putting `l-side-margins` on an element that already has classes, to avoid any unpredictable clashes.