Added Reinmar/ckeditor-descriptionlist plugin as a replacement of old definitionlist plugin.
Extracted mdn-link-launch plugin from old mdn-link.
MDN's enter key plugin isn't needed any more.

It contained special cases for definitionlist plugin. Current
descriptionlist plugin handles these cases by itself.
Extracted mdn-table-customization plugin from old mdn-table plugin.
Rebuilt CKEditor.