[IMP] point_of_sale: difference amount authorized on POS config

You can currently set a difference amount authorised in a cash journal
when a pos session is closed. This feature, allow a POS user to validate
a POS session having a difference between what you are supposed to have
in your cashbox and what you are really have. If the difference is too
big, you have to be POS manager to close the session.

We've improved the feature:

First we've moved the difference limit from the journal on POS config
and not on the journal as it is more a way that the POS is used than an
accounting configuration.

We've also added a wizard to warn a pos manager that the difference is
too big, because before the manager could always validate a session
without being warned that there can be a problem.

TASK-ID: 1934784