sends etag on immutable icing resources

[Martin Pool]

> curl -Ov https://code.launchpad.net/+icing/rev13265/lazr/build/lazr/assets/skins/sam/negative.png
(for instance)
shows that Launchpad sends an etag header on that resource, even though it ought to be immutable. I believe this is generally regarded as useless or possibly harmful, and client caches will behave better if they are just given a long expiry time.

I haven't demonstrated any actual performance cost from this, and it may not be measurable, but it does give a yellow flag in google page speed test.

(filed after discussion with gary)

See also bug 92492