Netlify-cms: Stop exposing `createClass` and `createElement` directly.

[tech4him1]

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.

[erquhart]

We should deprecate and update the CMS object to contain these references, agreed.

[tech4him1]

For deprecation, would you just suggest adding a console error or something more visible?

If a console error, would you make it a warning or an error (since errors sometimes throw build systems).

[nassdonald]

Just a heads up, there are a few code examples using these methods in the Netlify CMS docs.

[erquhart]

RE: @tech4him1's comment, a console warning should work for this.

[tech4him1]

Do we want these nested under a CMS.lib or just CMS? They are integral to how the CMS works, so maybe just on the main registry.

[stale[bot]]

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