[WIP] Adds default configuration for SonataAdmin frontend link

This is WIP because symfony-cmf/RoutingBundle#273 must be merged before.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | 
if you support Route objects too, we could enable the extension for them as well.