Allow NGINX to pass websocket requests to Workhorse
gitlab-workhorse!83 (merged) 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)