Bug 38309: Cannot delete additional fields

Bug 38309: Cannot delete additional fields

Deleting an additional field tries to do a GET of ?op=delete... which no
longer works with CSRF protection. It needs to be a POST with op=cud-delete.

Test plan:
 1. Administration - Additional fields, click any table name
 2. New field, fill in a name and Save
 3. In the row for your field, click Delete
 4. You have a blank page
 5. Apply patch, click the browser Back button, refresh the page
 6. In the row for your undeleted field, click Delete
 7. Your field is deleted

Sponsored-by: Chetco Community Public Library
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>