Changed logout routes to POST instead of GET

As per #3047.

Also made some SAML specific fixes:
- IDP initiated login was broken due to forced default session value.
  Double checked against OneLogin lib docs that this reverted logic was fine.
- Changed how the saml login flow works to use 'withoutMiddleware' on
  the route instead of hacking out the session driver. This was due to
  the array driver (previously used for the hack) no longer being
  considered non-persistent.

app/Auth/Access/Saml2Service.php

@@ -99,7 +99,7 @@ class Saml2Service
      * @throws JsonDebugException
      * @throws UserRegistrationException
      */
-    public function processAcsResponse(string $requestId, string $samlResponse): ?User
+    public function processAcsResponse(?string $requestId, string $samlResponse): ?User
     {
         // The SAML2 toolkit expects the response to be within the $_POST superglobal
         // so we need to manually put it back there at this point.