Temporary measure so we can cut a release from master while some Redis
integration issues are still being sorted out.

https://gitlab.com/gitlab-org/gitlab-workhorse/issues/113

https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/136

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.

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/33

The -relativeURL option existed for a while but was never included in
an official release.

This is a breaking API change. Instead of parsing repo path from the request
in gitlab-git-http-server, we now expect the auth backend to tell us the full
path to the repository. This change is needed to handle API download requests.