Purpose
=======
An employee and its current contract should have the same
calendar. Currently, if the contract's calendar is changed,
the employee's calendar is not changed.
In the same way, if a contract is created directly in `open`
state, the employee's calendar is not changed.
Finally, the calendar synchronization when the employee
contract is changed is only done in payroll, for no reason.
Specification
=============
Fix the three issues described above.
If the employee's calendar is nonetheless manually changed, a warning
should be displayed if there is a mismatch between both calendars.
Raphael Collet