WIP: set etags for static resources

[syphar]

A click too much created the PR, now I'll just leave it as WIP.

First step towards #1560.

I can't let go of the fact that Firefox is redownloading all that stuff from the CDN. After research, the next reason could be that we currently don't return ETags.

This PR implements this, first for:

1. our own static files
2. static resources returned from docs

Next steps after this then:

• figure out how we could generate a ETag for rustdoc pages, without generating them. ( note to myself: does MatchedRelease contain enough information for this?)

remaining TODO:

• research why Option<TypedHeader<IfNoneMatch>> never returns None, even when the header is missing