Bug 29153: Fix CodeMirror textareas to work with all languages

Bug 29153: Fix CodeMirror textareas to work with all languages

To test:
1. Go to Tools > HTML customizations
2. Open or create a new entry with CodeMirror. ( Edit with text editor )
3. CodeMirror doesn't work, notice a console error.
4. Apply patch
5. Try steps 1 & 2 again. The CodeMirror editor should now load.
6. Make sure it works in both the Default and English(en) tabs.
7. BONUS: install some other language packs and look at the tabs for that language, it should still work.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>