Fix the Admin getSubject() method to work with phpcr node IDs

The base Sonata Admin class has an overly strict regex limiting what can be a valid document ID to numbers and uppercase/lowercase A-F. This PR loosens the characters allowed, and calls absolutizePath on the ID before doing a find with the document manager.
can you document in the phpdoc why you overwrite this here? and say that valid ids are everything that DocumentManager::find accepts, thus also uuid?