Bug 31115: Add support for additional fields for invoices

Bug 31115: Add support for additional fields for invoices

This patch adds support for additional fields for invoices. A new option
is added to the 'Additional fields' admin page, for the 'aqinvoices'
table.

Adding/editing invoices now supports this additional fields.

To test:
1. Apply this patches
2. Verify the original test plan works
=> SUCCESS: It does!
3. Sign off :-D

Sponsored-by: The Research University in the Helmholtz Association (KIT)
Signed-off-by: Michaela Sieber <michaela.sieber@kit.edu>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>