[WIP] Adds default configuration for SonataAdmin frontend link

Relates to symfony-cmf/RoutingBundle#273

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |