Port grpc-web interop tests from old repo

I'm very pleased to see interop tests are now part of the official repo, and I can deprecate my old [interop test repo](https://github.com/johanbrandhorst/grpc-web-compatibility-test). However, there are still a few clients and proxies represented in this repo which haven't yet been ported across, for example:

* The Improbable grpc-web client.
* The Improbable Go grpc-web proxy (standalone and in-process).
* The [akka-grpc](https://github.com/akka/akka-grpc) Scala proxy (credit @timw),
* The [grpcwsgi](https://github.com/public/grpcWSGI) Python proxy (credit @public).

Fortunately, all of these should be quite easy to port, as they are already packaged in independent Dockerfiles, with a setup much like the one here.

It would also be nice if we could port the existing compatibility status matrix, since I think it gives a nice overview, and would be nice to carry forward.