edit/page: fix ckeditor [json] upload response

One incompatibility in commit v4.4-28-gdd363674c2 (2020-12-05)
[update remote ckeditor to version 4.15.1] after all, according to
https://ckeditor.com/docs/ckeditor4/latest/guide/dev_file_browser_api.html:

> Since CKEditor 4.9 all file uploads, including those initiated by the File
> Browser plugin, expect a JSON response