Bug 22659: Add save and continue button to additional-contents.tt

Bug 22659: Add save and continue button to additional-contents.tt

To test:
1. Apply patch and restart everything
2. Go to Tools > News and create some new additional content.
3. Notice a Save and continue button
4. Try saving and contining.
5. Make sure if you are using the CodeMirror editor that you are still in the CodeMirror editor
6. Try 2 - 5 again but with the wysiwyg editor, make sure when you save and continue you remain in the wysiwyg editor.
7. If you are saving and contining from News make sure you remain in News, when you are saving and contining from HTML customizations make sure you remain there.
8. Turn on the NewsLog system preference
9. With the NewsLog on make sure your content is being logged correctly when you sabe and continue.

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>