Enable recursive breadcrumbs in admin

[dbu]

We should make something like https://github.com/symfony-cmf/MenuBundle/blob/master/Admin/MenuNodeAdmin.php#L44-L81 possible for simple cms as well.

An admin extension can not do this, otherwise we could generalize it into one. Given that trees are inherent to PHCPR-ODM, we could build the feature into sonata admin phpcr-odm directly, however.