Move banner_notification into after_header block

This moves the banner_notification from the top of the content
block and into the bottom of the after_header block. This
has the effect of moving the campaign bar above the global
header bar - it looked odd when it was below it.

Arguably the banner notification should be above the user satisfaction
survey because it's more important, but I didn't want to make that
change now.
Update banner notification styles

- Do a bit of mobile first #buzzword
- Fix alignment to 960px page
Hide banner_notification conditionally on homepage

Whenever we hide the navigation, we should also hide the banner
notification.

Fixes #433.
Merge pull request #432 from alphagov/bugfix-campaign-header-bar

Move banner_notification into after_header block