Discount Types

[drewroberts]

I've been thinking we need to add Discount Types that function similar to Voucher Types (TIPOFF/vouchers#23) for restrictions on how discounts can be applied during the checkout process.

[pdbreen]

For both vouchers and discounts, going to propose a somewhat minimal common base table (code plus a few other TBD fields) with a morph to the class that knows how to determine if the voucher/discount applies to either a cart item or an entire cart as well as how to apply it. This allows any future voucher or discount logic to be a combination of additional data fields and code logic that knows how to interpret and apply them.

[drewroberts]

I like this approach. I look forward to seeing it in more detail. Thanks!