Fix AjaxAuthenticationSuccessHandler to better handle url generation (generates app_dev.php when necessary)

The DefaultAuthenticationSuccessHandler class handles URL generation like this :

```php
$this->httpUtils->createRedirectResponse($request, $this->determineTargetUrl($request));
```
Instead of just :
```php
$this->determineTargetUrl($request);
```

The createRedirectResponse method itself relies on the <code>HttpUtils::generateUri()</code> method that adds the <code>/app_dev.php/</code> when necessary.

There are 2 approaches to this problem.
1. Copy and paste the URL generation logic inside your child SuccessHandler
2. Use the RedirectResponse::getTargetUrl() method to get the generated URL

The second approach seems cleaner to me, let me know your opinion on this matter :)