Upgrade CKEditor to 4.12.1 (fixes memory leaks)

The latest 4.11 release of CKEditor (4.11.4) fixes a bunch of memory leaks. We should upgrade to this version.

Release announcement: https://ckeditor.com/blog/CKEditor-4.11.4-with-improved-memory-management-released/

Release notes: https://ckeditor.com/cke4/release-notes

Details on memory leaks fixed: https://github.com/ckeditor/ckeditor-dev/pull/2969