Replace unmaintained react-loadable package with loadable-components. (#1407)

* Replace react-loadable with two active alternatives.

`react-loadable` no longer has an active maintainer. Only personal forks are available for Webpack 4 + Babel 7 support.

* Remove react-universal-component.

Change recommendation to only `loadable-components` and link directly to the SSR documentation.

content/docs/code-splitting.md

@@ -115,7 +115,7 @@ parse the dynamic import syntax but is not transforming it. For that you will ne
 
 > Note:
 >
-> `React.lazy` and Suspense is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend [React Loadable](https://github.com/thejameskyle/react-loadable). It has a nice [guide for bundle splitting with server-side rendering](https://github.com/thejameskyle/react-loadable#------------server-side-rendering).
+> `React.lazy` and Suspense is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we recommend [Loadable Components](https://github.com/smooth-code/loadable-components). It has a nice [guide for bundle splitting with server-side rendering](https://github.com/smooth-code/loadable-components/blob/master/packages/server/README.md).
 
 The `React.lazy` function lets you render a dynamic import as a regular component.