Add number to unnumbered item pools

[jim]

Reservable items have a user-friendly number that identifies them (and is encoded in barcodes). We need to be able to assign a number to item pools that are uncounted as well. The trick here is that any database constraints won't be able to ensure uniqueness. Just doing that in Ruby via the validation system is probably OK though (we don't expect these values to be changed regularly after the items or pools are created).