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.
> You had to tweak it and add some Elgg specific stuff.

that is true for 1 line (the elgg.echo) :)

AMD is not possible due to how it has to be loaded, but i will convert it to a .js view