Move default CKEditor config to hippo-ckeditor project

The default configuration for CKEditor fields currently resides in the hippo-cms-richtext-ckeditor-frontend module in the hippo-cms project. Since that configuration needs to be shared with the content-service backend in the channel manager, and we don't want to create a direct dependency from the content-service on the CMS, the default config should be moved to a separate location shared by both the CMS and the Channel Manager. The hippo-ckeditor project on Github seems to most logical location.