[FIX] hr_holidays: better display of leave counter (opw 605016)

The number of leave requests left is based on the employee_id key in the context, when displaying the status of an hr.holidays we don't have the information in the name_get of the hr.holidays.status.
The fallback on the number of employees is done with the uid but we can not rely on it as well as the name_get on m2o field is done with superuser_id.