Allow NGINX to pass websocket requests to Workhorse
https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/83 adds a websocket endpoint to Workhorse. To use this, NGINX must be configured to pass through the Connection: upgrade
and Upgrade: websocket
headers if present. This already happens if we're proxying mattermost, so we just extend the mechanism to cover workhorse here.
Closes #1769 (closed)