Proxy to webpack dev-server in developmentMode

We will be having a new component in GDK that serves live-compiled javascript assets in development mode at the resource /assets/webpack. https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/237

For the sake of transparency to the GDK user I think it should be workhorse's job to proxy /assets/webpack to webpack-dev-server.

• add -webpackAddress option, default localhost:3809 (correct @mikegreiling ?)
• add a route in internal/upstream/routes.rb that only kicks in when developmentMode is set
• use httputil.ReverseProxy on this route.

@nick.thomas sound reasonable?