Increase Spotlight assets cache time to 1 month

When we deploy applications, we frequently see issues where the page
is rendered without assets.

Initially we thought Varnish was working incorrectly, but actually I
think the issue is that the files are not in the cache on both frontend
app servers, which means that we respond with a 404 in some cases.

If the file is in Varnish on only one frontend app server, and somebody
who does not have the page cached locally makes a request that hits
the other frontend app server, they'll get a 404.

An easy way to fix this is to increase the cache time to a month, which
we can do now because everything is being properly cachebusted.
Merge pull request #303 from alphagov/increase-spotlight-asset-cache-time

Increase Spotlight assets cache time to 1 month