django-ckeditor required for initial migrations, but is no longer used

I believe we've replaced django ckeditor with tinyMCE (it will be worth double-checking this assumption before tackling this ticket).

If that is the case, it would be great to:
* Clean django-ckeditor out of the older migration files
* Remove it from `requirements.txt`
* Ensure any changes to the migrations do not break existing installations of the ITK DB