Fixes error with unperisted dynamic route in sonata FrontendLinkExtension

Fixes a serious error at the ```\Symfony\Cmf\Bundle\RoutingBundle\Admin\Extension\FrontendLinkExtension``` when a new dynamic route is created in sonata admin:

```
An exception has been thrown during the rendering of a template ("Can not determine the prefix. 
Either this is a new, unpersisted document or the listener that calls setPrefix is not set up correctly.") 
in SonataAdminBundle:CRUD:base_edit.html.twig at line 34.
```

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony-cmf/cmf-sandbox#276
| License       | MIT
| Doc PR        | none
yes it is, you just need to adjust the config to make this extension being loaded into the admin (then the problem with namespace you now fixed would have been shown as test failure). when you do, you could add a check in the web test cases to check if the frontend link is actually there (except in the create case where the whole tab must be not there, which you could test as well just to be sure)