Draft: POC: Streaming diffs app

This is an example implementation of Streaming for the diffs tab of merge requests.

Don't be scared by an amount of changed lines, entire GitLab UI has been replaced with a fixed version to support SSR.

Also this MR is one of the reasons Streaming should make sense. 😄

How to run locally

1. Remove node_modules folder
2. Run bundle and yarn
3. Run GDK
4. Run webpack --watch --config=./config/webpack.ssr.config.js
5. Run node server.js when previous command compiles the bundle

Go to any merge request and open Changes tab.