Bug 29693: Make CodeMirror work again

Bug 29693: Make CodeMirror work again

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, you don't see line numbers or syntax highlighting
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.
8. Try 5 and 6 again but with Additional contents (News) instead of HTML customizations

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>