Add "Active" column to discount codes table

Created by: seancolsen

This makes it easier to manage discount codes when you have a whole lot of them and many of them are disabled.

Before

• Active discount codes are visually indistinguishable from inactive ones.
• There is no way to view a list of all active discount codes.

After

• A new column shows the status of each code.
• Users can sort by this column to show only the active codes.

Fixes #182 (closed)