Revert "Order invoice items by SaleItem.te_id"

Ordering by te_id had no effects. Instead, order by Sellable.code directly on
Domain and document it.

This reverts commit 4babbeda6eb9cd1a41381d90682bbc4e90aea9ad.

Change-Id: I27476e1388f86dfd083a2c0bd8ba88f2f670ebf5
Use batch selection on work orders

Change-Id: I3957ee1297b6ab7a185cf8e278a69b293e94664a