[IMP] hr_holidays: Clarify the value of manager_id

Currently the manager fields have this meaning:

 * manager_id: employee that approved the leave first;
 * manager_id2: employee that approved the leave secondly;

It makes no sense. Furthermore, on the Leaves Dashboard, when you group
by Manager, you group by first approver, which is also a nonsense.

This commit improves the behavior

 * rename manager_id and manager_id2 into first_approver_id and
   second_approver_id in order to understand their meaning
 * add a stored related field manager_id, readonly to avoid modifications
   on the employee
 * modify the views to only display the real employee manager, the other
   values are not valuable on the views
 * in code regularly a manager variable is used but is actually the
   current employee; a renaming is then applied

0 files changed, 0 insertions, 0 deletions