Stop exposing `createClass` and `createElement` directly.

React's `createClass` and `createElement` methods are exposed directly on the `Window` object by the CMS, for building extensions. This isn't very clean, and could possibly result in conflicts. Also, we have the user using the React class methods (`render`, `handleChange`) directly, which ties us to that specific implementation.

https://github.com/netlify/netlify-cms/blob/6ad6bfd094b78696744f87ffd8038656a2922f7a/src/index.js#L58-L62

Related to #571.

Start with src/index.js at lines 58-62 and read related issue #571. Determine how extension users should access CMS functionality without exposing React methods on Window; done means the direct createClass and createElement exposure and reliance on React class methods are removed while extension behavior remains supported.

Written by the indexing model from the issue text.