Move the GET /xxx/{id}/categories requests to the corresponding classes
(Contacts, Products, Thirdparties).
Move the addCustomerCategory() method to the Thirdparties class. Use the
POST verb for the request instead of GET because the request modifies
the state of the server.
Add a User parameter to the Product::delete() method to explicitly set
the user that performs the action instead of using a global variable.
In the calls to Product::delete() with a fetched object, remove the
deprecated parameter $id.