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/Http/Controllers/Auth/Saml2Controller.php

@@ -5,8 +5,7 @@ namespace BookStack\Http\Controllers\Auth;
 use BookStack\Auth\Access\Saml2Service;
 use BookStack\Http\Controllers\Controller;
 use Illuminate\Http\Request;
-use Illuminate\Support\Facades\Cache;
-use Str;
+use Illuminate\Support\Str;
 
 class Saml2Controller extends Controller
 {
@@ -79,11 +78,6 @@ class Saml2Controller extends Controller
      */
     public function startAcs(Request $request)
     {
-        // Note: This is a bit of a hack to prevent a session being stored
-        // on the response of this request. Within Laravel7+ this could instead
-        // be done via removing the StartSession middleware from the route.
-        config()->set('session.driver', 'array');
-
         $samlResponse = $request->get('SAMLResponse', null);
 
         if (empty($samlResponse)) {
@@ -114,7 +108,7 @@ class Saml2Controller extends Controller
             $samlResponse = decrypt(cache()->pull($cacheKey));
         } catch (\Exception $exception) {
         }
-        $requestId = session()->pull('saml2_request_id', 'unset');
+        $requestId = session()->pull('saml2_request_id', null);
 
         if (empty($acsId) || empty($samlResponse)) {
             $this->showErrorNotification(trans('errors.saml_fail_authed', ['system' => config('saml2.name')]));