Proxying websockets

In https://gitlab.com/gitlab-org/gitlab-ce/issues/22864 we propose adding a WebSocket to gitlab-workhorse to proxy terminal connections. gitlab-workhorse!83 (merged) adds support for that in workhorse.

However, the nginx configuration generated by omnibus-gitlab does not correctly proxy websockets as it stands.

Changes to the configuration will need propagating to gitlab-development-kit and gitlab-ce, both of which have nginx.conf files embedded.