Replace math in template with variable, smarty3 support

Replace math in template with variable, smarty3 support

The handling for math in smarty2 & 3 differs wrt precedence, when used
with a modifier. It's a bit tricky supporting both but in this
case we can simply rely on another key in the array that already exists
and is equivalent

Note that the membership online template is excluded deliberately for now
as it does not currently access the same lineItems variable
as all the other templates