Bug 33863: Update values in data that will be saved

Bug 33863: Update values in data that will be saved

We are not saving the values of the input but a representation of the
form, contains in $("#order_edit").data('row');

To acchieve that it's preferable to reuse _update_unitprice and call it
when #change_currency is modified.

Some code cleaning slipped into the patch, for readability purpose.

Signed-off-by: Michaela Sieber <michaela.sieber@kit.edu>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>