Add immutable directive to Cache-Control header

[lpsmith]

When serving up a Content-Addressed static asset, include immutable on the cache-control header

However, I did notice that when serving up a asset that is Content-Addressed and redirects are disabled, we are disabling all caching. We could modify Obelisk.Asset.Serve.Snap.serveAsset' to be less conservative and more precise on this regard: in doing so, we'd need to remember the whether or not the first call to serveAsset' before it recurses is an immutable or a redirect, and then use that instead of doRedirect on line 94.

I have:

• Based work on latest develop branch
• Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• Run the test suite: $(nix-build -A selftest --no-out-link)
• (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[3noch]

and redirects are disabled

How does this happen?

[lpsmith]

and redirects are disabled

How does this happen?

By calling either serveAssetsInPlace or serveAssetInPlace instead of serveAsset(s). I didn't look at how those particular routines are used (or not) in typical codebases, though.