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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• [ ] Review tests are passing in the browser. Open http://localhost:8001/. - 2 tests for SLDSCombobox fail("selects a menu item and scrolls when a letter key is pressed in read-only mode" and "selects menu items and scrolls when the down/up keys are pressed"), but they also did not pass after a clean install.(Same as previous PR
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REVIEWER checklist (do not remove)

• [ ] 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.
• [ ] Review the appropriate Storybook stories. Open http://localhost:9001/.
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• [ ] Review tests are passing in the browser. Open http://localhost:8001/.
• [ ] Review markup conforms to SLDS by looking at DOM snapshot strings.

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[interactivellama]

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

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