Add React PropTypes for Array and Object Props

[noi5e]

Part of a React rewrite of the comment system, see #9365.

See React docs on PropTypes. This PR adds PropType checking for array and object data that's passed down as props throughout the App.

BEFORE: check to see if currentUser is an object.

App.propTypes = {
  currentUser: PropTypes.object
};

AFTER: check to see if currentUser is an object AND typecheck its properties:

App.propTypes = {
  currentUser: PropTypes.shape({
    canModerate: PropTypes.bool,
    id: PropTypes.number,
    role: PropTypes.string,
    status: PropTypes.number
  })
};

This increases the long-term maintainability of this project. PropTypes help with debugging and ensure that the data the app receives is valid.

@jywarren mentioned writing functional tests to make sure the controller is sending valid data to the client... IMO PropTypes can easily substitute for functional tests! 😄

(Example: PropTypes logs an error to the console if data doesn't match expectations:)

---

(This issue is part of the larger Comment Editor Overhaul Project with Outreachy. Refer to Planning Issue #9069 for more context)

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[RuthNjeri]

Great job @noi5e, it looks good to me!

[jywarren]

Thank you so much @noi5e - great work!