Add logout endpoint to DRAPI's oauth system

The .well-known/openid-configuration in DRAPI does not show a logout endpoint. A client web application can use the /oauth/authorization and /oauth/token endpoints to authenticate and refresh the authorization token. A logout is currently only possible by removing a user_token property from the browser's local storage. But this does not work when the app does not run on the same domain as the authentication system.

By having an /oauth/logout endpoint DRAPI can take care of removing that local storage item, and the application does not have to know anything about the technical details of the authentication technicalities.