Don't use renderToStaticMarkup to render course progress.
Don't use renderToStaticMarkup to render course progress.

Stop using [renderToStaticMarkup](https://facebook.github.io/react/docs/top-level-api.html#reactdomserver.rendertostaticmarkup) to render course progress because it's not supposed to be used in client code, and has been [moved to ReactDOMServer in React 0.14](https://facebook.github.io/react/blog/2015/10/07/react-v0.14.html#two-packages-react-and-react-dom).

Instead, create and prepend a mount point and render into that using regular ReactDOM.render.

This may affect an [old TODO from 6343](https://github.com/code-dot-org/code-dot-org/pull/6343#discussion_r49917074).  FYI @joshlory.

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