[OIDC] `post_logout_redirect_uri` is wrong when `ORG_IN_PATH=true`

[vviers]

Describe the current behavior

The URL after loging out of Grist is https://{URL}/o/{ORG}/signed-out when it should be something like https://{URL}/signed-out

Indeed, the OIDC spec states that post_logout_redirect_uri "MUST have been previously registered with the OP, either using the post_logout_redirect_uris Registration parameter or via another mechanism."

However, it is impossible to register all possible, orgs-specific, logout urls. You can't register a wildcarded URL either such as https://{URL}/o/*/signed-out

Steps to reproduce

1. Create a Grist instance with ORG_IN_PATH set to true
2. Log in
3. Navigate to a team site
4. Log out

Describe the expected behavior

Logout URL should not depend on which team site you sign out from.

https://{URL}/signed-out seems sensible as a value.

Where have you encountered this bug?

• [ ] On docs.getgrist.com
• [X] On a self-hosted instance

Instance information (when self-hosting only)

• Grist instance:
  • Version: 1.1.18
  • Installation mode: k8s
  • Architecture: multi-workers