[IMP] hr_timesheet: convert onchange to stored-editable computed fields

The fields 'project_id' and 'user_id' have been converted to
stored-editable computed fields.

Convert the 'allow_timesheets' field.

Improvement of the _compute_task_id method to check the task_id field before using it.

Update hr_timesheet tests by removing calls to onchange methods because
now, compute methods are always called.

Get back the onchange on the 'project_id' to compute the task_id field due to a complex
bug found in sale_timesheet tests.