Use Illuminate Session component instead of Symfony

Symfony's component relies on PHP's native session functionality, which
is not ideal. It automatically sets its own cookie headers, resulting in
this issue: https://github.com/flarum/core/issues/1084#issuecomment-364569953

The Illuminate component is more powerful and has a simpler API for
extension with other drivers and such, and fits in nicely with other
components we use (the majority of which are from Illuminate).

framework/core/src/Frontend/FrontendView.php

@@ -464,7 +464,7 @@ class FrontendView
 
         return [
             'userId' => $actor->id,
-            'csrfToken' => $session->get('csrf_token')
+            'csrfToken' => $session->token()
         ];
     }