Clean up styling of campaign page on mobile devices

This PR fixes a couple of issues with the campaign show page when viewed on a smaller screen. The original bug ticked is here: https://www.pivotaltracker.com/story/show/73141030, but this has gone a little beyond the scope of that bug.

I've changed the scoping of some of the styling here and moved some stuff into a more logical order, making this PR's diff rather difficult to read, but here are the changes:

• Content no longer loses it's margin when viewed between 900px - 1000px wide

• Margin applied to section#landing instead of article.campaign

• Cleaned up behaviour of header / org logo on smaller screens
	(now only tries to render them left / right on desktop)

• Leading header image is no longer full window width on small browsers

• Leading header image no longer gets really tall on small browsers

• Some css in _campaigns.scss was unscoped (eg .organisation), these are
  now scoped

• Remove #extra_links rules (no longer in use)