Use Cache-Control: immutable on images

[François Marier]

Since no browser currently supports "stale-white-revalidate" (added in bug 1406843), we should switch to using "immutable":

  https://hacks.mozilla.org/2017/01/using-immutable-caching-to-speed-up-the-web/
  https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control#Revalidation_and_reloading

Not only will it speed up load time when the image is cached, but it will also reducing server bandwidth, by eliminating 304 responses, when reloading pages or displaying the save images again.

[François Marier (fmarier)]

Actually, I misunderstood how "immutable". This is for versioned URLs only. The resources that are cached as "immutable" will NEVER expire.

If we used this to serve avatars, it would prevent users from being able to change their avatars.