Increase cache duration for Spotlight assets

I'd like to increase the `Expires` header as an indication that I'm more confident in our ability to cachebust than I was previously.

Currently the only assets that won't be cachebusted are images in stylesheets and the govuk_template - the govuk_template will be cachebusted as soon as https://github.com/alphagov/govuk_template/pull/50 is merged and we update to it.

We're still seeing some odd issues whereby the page is rendered with no CSS. I think this is because the browser caches the HTML for half an hour which then references CSS that no longer exists. I think we can solve this by increasing the `Expires` header further and putting the assets vhost behind Varnish. Perhaps we should also investigate keeping old assets on the server for a period of time.