Make gatekeeper's auto_auth option a form parameter specified via middleware configuration

Follow up for https://gitlab.slack.com/archives/C02Q93U8J07/p1695036196197079

The problem is that Traefik does forward auth differently than ingress-nginx, and we need to construct redirect URLs using the headers passed by traefik. On top of that some of our API endpoints had incorrectly defined auth-auth behaviour. This MR addresses both issues.

Tested using devvm.