Set `validPostLogoutRedirectUris` on workspace initialization

[aaronschweig]

In order to properly support logout flows in the portal, we need to be sure to set this value.

e.g.

- http://<org-name>.<portal base domain>/logout*

Todos

• Check if the organization-idp chart already allows setting logout urls - if not add
• Set the corresponding value on realm initilization
• (Migrate the existing realms to support this value - may be solved by regular reconciliation of organizations)