Clean up styling of campaign page on mobile devices

• Content no longer loses it's margin when viewed between 900px - 1000px wide

• Cleaned up behaviour of header / org logo on smaller screens
	(now only tries to render them left / right on desktop)

• Margin applied to section#landing instead of article.campaign

• 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)
Merge pull request #623 from alphagov/clean-up-campaign-styling

Clean up styling of campaign page on mobile devices