[orders] implement "sort" command

[myk002]

I have found that orders with daily frequency will prevent lower priority orders for the same types of items from ever getting scheduled. This is a problem when "workflow"-type orders that keep a small stock of a thing will prevent large one time orders for that same type of thing, significantly slowing down the project that the large order was created for.

This PR implements the sort command for the orders plugin, which can be called with repeat to automatically keep a fort's orders sorted so daily-frequency orders don't "shadow" any other orders.