hr_car: Add field internal number and active

Each car should have the field internal number or similar. If you have a fleet like us, we have our own (short) number, that is written on the car, on the keys and on the planning as well as in the brains of the drivers. This internal number should be a string field, because some fleets useing letters as well. This field should be the first column in the list.
Because a car does not live for ever, the car should also have a flag called active. If a car is no longer active, then it could be filtered from the list. Similar functionality as for hr persons.