Add multiple table subtotals to specify which columns we want included in that amount, and reference that subtotal as a variable in the document text

Right now, by default, when you add a subtotal, it just includes all the price columns, but we want to be able to click on it and enable/disable the columns you do or don't want in that subtotal

We also want the discount column to be configurable similarly, where we can specify which columns that discount applies to, because right now it applies to the whole row

We ultimately just need the smart fields to be editable so we can actually specify which columns are included in the calculations.