Bug 15326: Using AdditionalContents for custom pages on OPAC

Bug 15326: Using AdditionalContents for custom pages on OPAC

This page utilises the Additional Contents feature to add custom pages
to the staff client and the OPAC in the user's desired language.

To test:
1. Apply patch and restart services
2. In the staff client, go to Tools, then go to the new 'Pages' page
3. Add a new page to display on both the staff client and OPAC.
4. Confirm the URLs in the Page URL column work as expected.
5. Confirm that any pages made for the staff client only do not show in
the OPAC, and that any pages made for the OPAC only do not show in the
staff client.
6. Confirm that pages do not show on the OPAC main page where news items
show. Confirm news items show on the OPAC main page as expected.

Sponsored-by: Chartered Accountants Australia and New Zealand
Signed-off-by: Thibault Kero <thibault.keromnes@univ-paris8.fr>
Signed-off-by: Lisette Scheer <lisettePalouse+Koha@gmail.com>
Signed-off-by: Lisette Scheer <lisettePalouse+Koha@gmail.com>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>