fix(plugins): ckeditor now prevents image drag/drop/paste in editor

fixes #4970

code snippet is added as an external source, so it will not interfere with the default ckeditor code folder

TODOs:
 - [x] Change message to `fix(ckeditor)`
 - [ ] Move custom JS out of `vendors` and into an AMD file.
extraPlugins loads the plugin, ckeditor.js just registers it. Similar like elgg_register_js and elgg_load_js...