Redirect to target_path after successful registration/connection

Missing use of the default_target_path after a successful operation.
Because the target_path stored in the session is most likely an absolute path, this won't play nice with Symfony's per-environment front-controllers (eg: /app_dev.php).

The service 'security.http_utils' (<code>Symfony\Component\Security\Http\HttpUtils</code>) has a method <code>createRedirectResponse()</code> which handles this correctly. Unfortunately it is a private service and can only be injected into other services.