Media widgets

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes

This fixes route generation for media admin edit in ``media_widgets.html.twig`` when using custom ``Media`` class (extending ``\Sonata\MediaBundle\Entity\BaseMedia``) instead of default ``\Application\Sonata\MediaBundle\Entity\Media``.

```yaml
# app/config.yml

sonata_media:
    class:
        media:              Acme\DemoBundle\Entity\Media
```