Custom fields: configuration hierarchy has changed

Global themes are now stored under twig > form > resources instead of twig > form_themes:

```yml
        # app/config/config.yml
        twig:
            form_themes:
                - 'AcmeDemoBundle:Form:fields.html.twig'
            # ...
```

Becomes

```yml
        # app/config/config.yml
        twig:
            form:
               resources:
                  - 'AcmeDemoBundle:Form:fields.html.twig'
               # ...
```