Reduce the size of the Admin class

A colleague of mine told me that the SRP was not taken into account when building this class. He was not wrong. This PR tries to move a lot of code out of the `Admin` class. The resulting component, named `BreadcrumbBuilder`, is still highly dependent on it though, because there are many things that need to be called : 

- the menu factory
- the label translator strategy
- the translation method
- the route generator
- the classname label getter
- the current child admin getter
- the routing methods
- the request getter
- the toString method
- the subject getter
This class should be a service, so stateless. The Admin instance should hold configuration parameters.