[UX] CKEditor: Include the Balloon Toolbar plugin introduced in 4.8, and start using it.

• [x] This feature was introduced in CKEditor 4.8, so we will need to update it first (#3355).

https://ckeditor.com/docs/ckeditor4/latest/guide/dev_balloontoolbar.html

This feature was introduced in CKEditor 4.8. It is provided through the Balloon Toolbar plugin that is not included in any CKEditor preset available from the Download site and needs to be added to your custom build with online builder.

This plugin adds a possibility to display a toolbar, pointing at a particular element in the editor content.