Improve caching

Figure out best response for incoming HTTP headers that affect caching. At this moment resources cached by CloudFront seem to get refreshed:

Update cache response headers so that resources stay cached longer in our AWS Cloudfront.

TODO

Return Not modified when (see #clientCacheValid()).

• Client header Etag matches
• Header If-Modified-Since matches lastModifiedDate
• Other headers?
• Return Not modified HTTPS status as soon as possible in code
• Consider updating maxage

Start with code in RepositoryDownloadController v1, then check all server code that now returns cache headers and add better cache-handling when possible.