Bug 31254: Add additional fields for accountlines

Bug 31254: Add additional fields for accountlines

Test plan:
1. Go to Admin » Additional fields
   There are two new categories: "Account lines (credit)" and
   "Account lines (debit)"
2. Create fields for both categories, with and without an authorized
   value category
3. Go to a user's accounting page
4. Create a manual invoice. Verify that all "debit" fields are there,
   put a value in them and save
5. Create a manual credit. Verify that all "credit" fields are there,
   put a value in them and save
6. Make a payment. Verify that all "credit" fields are there, put a
   value in them and save
7. Go to the transactions tab, click on the "Details" button for the
   lines you just created and verify that the additional fields are
   there

Signed-off-by: Emmanuel Bétemps <e.betemps@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>