Don't use margin + 100% width for campaign header

As this causes the page to be larger than the screen.

This was causing an issue, most noticeable on a tablet, where the either a white margin was being displayed on the right of the page or the user could scroll slightly off the right of the page.

Only affects campaign pages when viewed at a tablet size.

Pivotal story: https://www.pivotaltracker.com/story/show/72330640

The pivotal ticket suggests that it would be a good idea to use grid classes to fix this issue. I haven't gone there as a tiny tweak would fix the issue and there are plans to rationalise css between apps anyway.