Enable single sign out when using OpenID Connect

[MJochim]

This allows the administrator to add the setting OAUTH_END_SESSION_URL. If they do, when users log out, Seahub will try to log them out of their SSO session globally. This is a standard feature of OpenID Connect (so strictly speaking it’s not OAuth). See the draft for session management (technically it’s a draft and not yet a standard, but it is already deployed in OIDC software).

One problem is that the http_or_https variable is not set correctly in some cases (when deploying behind a reverse proxy that terminates the TLS connection). This is very likely handled correctly in other places in Seahub‘s code but I don’t know how.

[freeplant]

We will give it a check.