Move WFC's "MAD Coupon" into a dedicated plugin. It was one of three
things (with taxTotal and fsTaxExempt) calculated by the subtotals view
and I want to eventually get rid of that view.

I revised the beta section of Database::getsubtotals() slightly after
some testing. I believe taxTotal and fsTaxExempt are both assigned
correctly now using taxView rather than subtotals but I want to do
more testing.