Bug 34921: Tabs on Additional Content page need space above

Bug 34921: Tabs on Additional Content page need space above

This patch adds a separator element, <hr /> between the additional
contents form and the tabs for visual clarity. Using an element is a
little hackish but it's a simple solution.

To test, apply the patch and go to Tools -> News -> New entry.

There should be a clear gap between the top form and the tabs below.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 0c74f429bcc5e05fc2444ce47349c9905c5fc448)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>