Supports absolute redirects being used in the FormController behaviour.
If the form_config.yaml specifies
```twig
create:
redirect: https://api.example.com/oauth/authorize
```
Then the behaviour will now properly redirect the user to the URL provided where previously it would redirect to a url along the lines of `october.example.com/backend/https://api.example.com/oauth/authorize`. Relative backend redirect URLs are unchanged.
9840ff228f