Added s-maxage, remove public

i. Our CDNs will use 's-maxage' to cache the resources.

ii. The clients will use the 'max-age' value.

iii. When we switch to Varnish (on a single domain) we either need to remove 's-maxage' on outbound requests or add the 'private' declaration.

In this regard I thinking the 'public' declaration is spurious.
Merge pull request #1065 from guardian/smaxage

Added s-maxage, remove public
Removed duplicate tests (cache-control is tested adequately in common)
Merge branch 'master' of https://github.com/guardian/frontend
Remove extraneous type attribute on analytics script tag
Merge remote-tracking branch 'origin/master' into local-nav
Updated CTA for secondary nav
Fix border colour for secondary nav
Top alignment fix
Fix height of the nav bars