Bug 15774: Add additional fields to order baskets

Bug 15774: Add additional fields to order baskets

This also moves the admin page for additional fields for all tables to a
single common screen, and factors out display/input parsing logic.

Test plan:
  1. Create an additional field for a subscription (under Serials -> Add
     subscription fields).
  2. Apply patch.
  3. Visit Additional fields under administration, and verify that
     the field created above still shows under the list for the
     subscription table.
  4. Create at least four fields for aqbasket for each combination of
     searchable/not-searchable and with/without an authorized value.
  5. Create an order basket, and verify that all fields are visible and
     correctly save.
  6. Edit the basket, verifying that changes to these additional fields
     are saved.
  7. Add an order to the basket (contents are irrelevant).
  8. Go to advanced search within acquisitions.
  9. Verify that only the searchable fields show in the form, and that
     their contents may be searched.

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>