FEATURE: Add logout functionality to SSO Provider protocol (#8816)

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.

spec/requests/session_controller_spec.rb

@@ -1105,6 +1105,32 @@ RSpec.describe SessionController do
         expect(sso2.profile_background_url).to start_with(SiteSetting.s3_cdn_url)
         expect(sso2.card_background_url).to start_with(SiteSetting.s3_cdn_url)
       end
+
+      it "successfully logs out and redirects user to return_sso_url when the user is logged in" do
+        sign_in(@user)
+
+        @sso.logout = true
+        get "/session/sso_provider", params: Rack::Utils.parse_query(@sso.payload("secretForOverRainbow"))
+
+        location = response.header["Location"]
+        expect(location).to match(/^http:\/\/somewhere.over.rainbow\/sso$/)
+
+        expect(response.status).to eq(302)
+        expect(session[:current_user_id]).to be_blank
+        expect(response.cookies["_t"]).to be_blank
+      end
+
+      it "successfully logs out and redirects user to return_sso_url when the user is not logged in" do
+        @sso.logout = true
+        get "/session/sso_provider", params: Rack::Utils.parse_query(@sso.payload("secretForOverRainbow"))
+
+        location = response.header["Location"]
+        expect(location).to match(/^http:\/\/somewhere.over.rainbow\/sso$/)
+
+        expect(response.status).to eq(302)
+        expect(session[:current_user_id]).to be_blank
+        expect(response.cookies["_t"]).to be_blank
+      end
     end
   end