Add CKEditor 5 stylesheet for Rivet

Problem/Motivation

CKEditor in Drupal backend should style Rivet classes.

Proposed resolution

Add a ckeditor5-stylesheets: entry in rivet.info.yml

(for now) use the KISS approach by simply importing the correct rivet stylesheet from CDN.

Most of the relevant styles are scoped with the .rvt- prefix, and only a few html tags have styles (eg figcaption), which in most cases would be overridden by the admin theme or ckeditor styles anyway.

Later we can look at doing more fancy postcss processing to import only the relevant styles for typical prose editing in CKEditor, eg replace .rvt-prose with .ck.ck-content, among other enhancements.

Remaining tasks

User interface changes

API changes

Data model changes