replace usage of findDOMNode with creating selfRef in constructor

[DNiederweis]

Fixes # Deprecation warning in console for findDOMNode in StrictMode.

Additional description

findDOMNode is deprecated in StrictMode as it "breaks abstraction levels" and "creates a refactoring hazard" - React Documentation. The React documentation instead encourages the use of references.

This pull request follows this advice and creates a self-reference in the constructor(selfRef). This reference is then used in componentDidUpdate when it is checked whether the modal is closing. If it is, then instead of using findDOMNode to get a reference to the modal and using that for unmounting and removing from the DOM, it instead uses the selfRef now.

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• CircleCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
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[interactivellama]

Thanks. This is legacy code that should be not be used, but is still here for backwards support.

[welcome]

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