Closes#20084
This is the flow that this implements:
1. A logged-in user clicks "Logout".
2. If they didn't auth via SAML, just do normal logout. Otherwise:
3. Form a LogoutRequest and redirect the user to
https://idp.example.com/slo-endpoint?SAMLRequest=<LogoutRequest here>
4. The IdP validates the LogoutRequest, terminates its own user session
and redirects the user to
https://thezuliporg.example.com/complete/saml/?SAMLRequest=<LogoutResponse>
with the appropriate LogoutResponse. In case of failure, the
LogoutResponse is expected to express that.
5. Zulip validates the LogoutResponse and if the response is a success
response, it executes the regular Zulip logout and the full flow is
finished.
Mateusz Mandera