Bug 16351: Making invoice number a required field

Bug 16351: Making invoice number a required field

EDIT: Added 'validated' class to form and removed 'Required' span (became unnecessary)

To test:
1) Go to Acqui -> Do a vendor search
2) Receive a shipment
3) Attempt to click 'Next' without specifying invoice number. Notice error.
4) Go back to vendor search then apply patch. Click Receive shipment
5) Notice you cannot click 'Next' until you put something in invoice number.

Sponsored-by: Catalyst IT
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>