Fix auto-increment invoice ID skipping numbers

[nymous]

When generating an invoice we request the Setting#next_invoice_id!, but this is committed immediately to the database even if the invoice is invalid and the transaction rolled back (contrary to when we assign an IP to a machine, both models get saved in the same transaction, but Rails knows they are related).
This means we generate invoice IDs with "holes" in the sequence, which is illegal.

When we fix this, we should definitely add some tests for it (in test/models/setting_test.rb?)