Implement "Rummage Sale" to sell off expiring inventory items at low cost

[jniles]

We have the backend tables to allow an enterprise to sell off large quantities of stock (in case they are expiring). We called this operation "rummage sale". While this isn't a great name, we can try to think of something better in the future.

Unfortunately, we do not yet have a module to actually do rummage sales. I propose we implement one. It seems like functionality any good enterprise should have.