Merge branch 'nick-kube-proxy' into 'master'

Handle environments/:id/terminal.ws, providing access to terminal websockets

A GitLab environment may expose a terminal connection for out-of-band access. Workhorse is responsible for providing a websocket connection to the terminal if present. 

It authenticates the user and retrieves connection details from GitLab using the environments/:id/terminal.ws/authorize endpoint, and sets up a proxy to the terminal provider, converting from the remote's subprotocol to a common format.

Authentication is periodically re-done, and the connection will be broken if it fails, or if the connection details change in any way.

This MR includes support for the `channel.k8s.io` websocket subprotocol, allowing connections to be made to Kubernetes / OpenShift containers. 

Based on top of (some of) !72 

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/22864

See merge request !83