User interaction during single logout

Traditionally, CATS has not permitted an IDP/OP to interact with the user during single logout. I.e.

> ...deployments MUST NOT deploy support for user intervention governing the choice of propagating logout to other SPs, or limiting the operation to the Identity Provider. At all times, a Single Logout Request will generate a global logout for the principal’s session.

The newly finalized OIDC RP-Initiated logout spec takes the opposite approach...

>At the Logout Endpoint, the OP SHOULD ask the End-User whether to log out of the OP as well. Furthermore, the OP MUST ask the End-User this question if an id_token_hint was not provided or if the supplied ID Token does not belong to the current OP session with the RP and/or currently logged in End-User. If the End-User says "yes", then the OP MUST log out the End-User.

Do we want to reconsider our approach or stick with our current approach?

Start with issue #40 and compare the existing CATS single-logout requirement with the quoted OIDC RP-Initiated Logout behavior. No implementation files or tests are identified in the issue. Done means reaching and recording a decision on whether user interaction during single logout should be supported.

Written by the indexing model from the issue text.