Refactor <StatusAlert />

https://github.com/edx/paragon/blob/master/src/StatusAlert

1. The component is technically both controlled AND uncontrolled right now. We should refactor to make this a controlled component (see https://stackoverflow.com/a/42522792 for more details).
2. Uses a deprecated React lifecycle method (i.e., componentWillReceiveProps). Instead, we should use componentDidUpdate (or potentially removed if it's not necessary after switching to a controlled component).
3. The content to show in the status alert should be passed in as children instead of as prop. For example:

4. A common use case for content within StatusAlert is to include an icon. We could add an icon prop to pass in an element to use for an icon (e.g., FontAwesomeIcon).