This commit adds support for an optional "logout" parameter in the
payload of the /session/sso_provider endpoint. If an SSO Consumer
adds a "logout=true" parameter to the encoded/signed "sso" payload,
then Discourse will treat the request as a logout request instead
of an authentication request. The logout flow works something like
this:
* User requests logout at SSO-Consumer site (e.g., clicks "Log me out!"
on web browser).
* SSO-Consumer site does whatever it does to destroy User's session on
the SSO-Consumer site.
* SSO-Consumer then redirects browser to the Discourse sso_provider
endpoint, with a signed request bearing "logout=true" in addition
to the usual nonce and the "return_sso_url".
* Discourse destroys User's discourse session and redirects browser back
to the "return_sso_url".
* SSO-Consumer site does whatever it does --- notably, it cannot request
SSO credentials from Discourse without the User being prompted to login
again.
ad2aa7b52c