Bug 15164 - Allow editing of the invoice number after initial saving

Bug 15164 - Allow editing of the invoice number after initial saving

When you notice a typo in your invoice number after finishing with the
receiving process, the only way to correct it right now is in the
database - or by undoing all the steps and starting over.

It would be nice if the invoice number could be edited.

Test Plan:
1) Apply this patch
2) Browse to acqui/invoice.pl for an invoice
3) Note the new "Invoice number" field contains the existing invoice
4) Alter the invoice number
5) Save the invoice
6) Note the invoice number has been updated
7) Edit the invoice again
8) Attempt to save the invoice with an empty invoice number field
9) Note that you cannot save the invoice without having an invoice number

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>