Support for statically compressed brotli content-encoding

Solves #125 (closed)

Same rules of !25 (merged) apply: To use it, static files that are to be served compressed must be present at the same location as the uncompressed file, with the extension .br.

Unlike compress/gzip, Brotli is not part of the go standard library. I've added https://pkg.go.dev/github.com/andybalholm/brotli@v1.0.0 for the tests.

Brotli has higher priority than gzip as long as the client accepts both.

Given how widely deployed it is, it should improve overall performance of gitlab pages.