Ckeditor update updates

Supercedes https://github.com/mozilla/kuma/pull/2921

On top of the original contribution, I added:

1.  The built files.  There's no reason to automate this now.
2.  Documentation on how to build.

This should make CKE work "immediately" - i.e. you check out this branch and CKE works when you hit the new/edit/translate pages.

If this "approved", we *DO NOT MERGE YET* but push this branch to mozilla/master and put it on staging.