[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        | 
and i wonder if we should call the menu point just "Frontend" and have a `title` attribute on the menu with the longer text. if that is possible with knp menu easily...