hr_payroll_account : allow to use employees' individual accounts

This is a proposal for issue #4491.

In the accounting tab of the salary rules, for both debit and credit accounts, it allows to select either "employee payable account", "employee receivable account", or "Select an account below".
If the choice is set to the employee payable or receivable account, then the employee's account is used during the creation of the accounting move.
If the choice is set to "Select an account below", the standard field is unhidden and one can choose an account like it is currently.

This change is supposed to be backward compatible, but there is no way to be sure because there are no tests for the accounting part of this module. I've enhanced the YML test to create an employee account, use it in the Net rule and check that there is a move line using this account in the resulting move.