Bug 10390: Add ability to delete empty invoices

There is currently no way to delete unused invoices (for example,
invoices created by mistake), and there really should be, since errors
and absent-mindedness can result in numerous empty invoices over the
course of years.

To test:
1) Apply patch.
2) Create three invoices in the Acquisitions module. For one of them,
   receive at least one item. For the other two, do not receive any
   items.
3) View one of the invoices that does not have any items on it.
4) Try to delete it. This should succeed.
5) View the invoice that has an item. There should not be any option
   to delete it.
6) Do an invoice search that brings up the other invoice with no items
   on it. Try to delete it from the results page. This should succeed.
7) Run the unit test:
   > prove t/Acquisition/Invoice.t
8) Sign off.