3.0 Checklist

Collection of breaking changes that are waiting for 3.0 (please comment if you have others):

- Drop "cms.js" files in distribution
- Manual init without a global flag (maybe config based? or different package?)
- Drop all processing of static characters in `slug` config template (#2006)
- Move `codemirror` to `netlify-cms`, as peer dep of `netlify-cms-app`?
- Convert registry `editorComponents` to object instead of Map (or implement stateful extension registration)
- Process `window.CMS_CONFIG` the same as any other config entry point.

**Deprecated in 2.x**
- Date widget (#2615)