Public uploads: Set an HTTP expiry time of 4 hours

This was set to 15 minutes by using the value for
`Whitehall.default_cache_max_age`. For public uploads we want to cache
the result in our CDN for longer to remove some of the load we create
on our own application servers.
Define cache max ages together
Set the cache max age time in a single file
Merge pull request #1723 from alphagov/increase-expires-header-for-public-uploads

Public uploads: Set an HTTP expiry time of 4 hours