Implement proxy authentication for the configuration server

[mariusoe]

For easier usage of the configuration server's web-UI, the configuration server should be able to support a proxy authentication.

If the config server is used behind a reverse proxy, which is able to authenticate the current user, it should be possible to pass this information to the configuration server. The config server should be able to login a user which is specified in a specific header field, set by the reverse proxy.

This function should be similar to the "Auth Proxy" function of Grafana. See: https://grafana.com/docs/grafana/latest/auth/auth-proxy/