[WIP] Adds link to the frontend page in SonataAdmin

SonataAdmin extension which adds a link to the frontend page on documents with a route.

Default configuration for the extension here: symfony-cmf/cmf-sandbox#276

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/116
| License       | MIT
| Doc PR        | 
we could add `&& !$subject instanceof Route` (with  Symfony\Component\Routing\Route). the generator can also handle a Route object. then we can use the extension on editing routes as well.