An an API for editor configuration

Problem/Motivation

At present editor configuration is found in drupalSettings.
We don't want the LayoutEditor to be aware of editor configuration, so passing it via props is not a good idea.
Instead, a CKEditor component should be able to fetch the config it needs via an API.

Steps to reproduce

Proposed resolution

Add an API endpoint for editor (CKEditor) configuration.

Remaining tasks

User interface changes

API changes

Data model changes