Bug 28006: Restore "Additional fields" on navigation menu

Bug 28006: Restore "Additional fields" on navigation menu

This patch adds a link to the "Additional fields" page in Administration
to the sidebar menu shown on Serials pages.

To test, apply the patch and go to Serials.

- In the sidebar menu you should see a "Manage subscription fields"
  link.
- The link should take you directly to the "Additional fields for
  'subscription'" page.

Signed-off-by: Amit Gupta <amitddng135@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>