Fix mapping quantities to selected lines

**I need this fix in 0.3 and 0.4 branches.**

Before fix `selected_line` list depends on selected checkboxes, but we always get all `selected_line_qty` regardless of the checkboxes. So we can end up having two lists of different length which are then zipped. So when you have two order lines on the list and you check only second checkbox you will get incorrect mapping of quantities to lines.

This view should probably use formset for order lines and form for actions. But I didn't want to make a revolution here as there are probably few other project that could get incompatible with this changes.