Bug 26121: Add z-index for CodeMirror

Bug 26121: Add z-index for CodeMirror

To test:
1. Make sure the system preference 'NewsToolEditor' is set to CodeMirror
2. Go add or edit some News content.
3. Add some content to the News section
4. Use the datepicker to pick a Publication date/Expiration date
5. Notice its very hard to see your dates overlapping with CodeMirror.
6. Apply patch
7. Much easier to see the dates!

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>