Send 'Cache-Control' header for immutable assets

Instruct clients to cache assets with a hash as part of the file name for up to 6 month and to do not send any requests for them, even when reloads are performed.

immutable is an extension supported by Firefox and Safari. Chrome does not support it but uses heuristics to apply a similar behavior.

Because the service worker handles caching for browsers, these changes should not have any effect on them as long as the service worker is active. The header is instead meant for reverse proxies.