Mark plone.stableResource responses as immutable

[davisagli]

The plone.stableResource ruleset sets a Cache-Control header with a long max-age (31536000) to allow caching for up to a year. But if the client force-reloads so that a request is made with Cache-Control: nocache, the resource will still be refetched.

Looks like it might be possible to avoid that by including immutable in the response's Cache-Control header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching#avoiding_revalidation