Support for OIDC id_token_hint in Logout URL

When using OIDC providers such as Keycloak, there is a need to include the id_token_hint query parameter of the logout URL. Some discussion on this behavior is [available here](



One way to support this parameter is for Outline allow a template substitution in the OIDC_LOGOUT_URI string. This is the approach [OAuth2 Proxy has taken]( to enable it. Whenever logout is initiated by a user from Outline, outline can place the users id_token value into the query parameter if the logout URI contains the template for it.