Bug 35300: Add page-section to invoice files page

Bug 35300: Add page-section to invoice files page

To test:
* Enable AcqEnableFiles system preference
* Go to acquisitions
* Search for a vendor and receive shipment
* Enter an invoice number and create new invoice
* Finish receive
* Click on 'manage invice files' link
* Upload a sample file
* Verify the table is missing the usual white background
* Apply patch
* Verify the the table now displays with the usual white
  background

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>