Bug 29113: Hide code for additional contents and generate it

Bug 29113: Hide code for additional contents and generate it

additional_contents.code is used to group DB rows together. Each row
represent one content in a given language, and the code is used to know
they are translation of the lang='default' one.

It's not really useful for the end user and we could hide it and
generate it.

Test plan:
Create/Edit/Delete additional contents (news and HTML customizations)
and confirm that they are correctly grouped together.
You need several languages installed to test this patch correctly.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>