Revert to older version of TinyMCE.

We had issues related to Cloudfront and file hashing. More time is needed for investigation.

@rocha Here is the revert of the TinyMCE version.

@andy-armstrong Do you want to code review? It's not an easy thing to review, though perhaps you could check out the branch and do some quick manual testing with the HTML component (not that I haven't already done that...).

@frrrances Also tagging you as an interested party.

This PR reverts #2657.