Add the Admin extension that enables editing HTML attributes

Contains an interface that encapsulates the attributes related methods for the menuNode.
Relies on burgov/key-value-form-bundle for the admin.
That sounds like a good solution. 

1. Remove the service if the burgov is not installed
2. Get rid of the explicit enabled config since it is unnecessary and the user is required to add the extension to the sonata_media config manually anyway so it doesnt actually enable the extension.