move from string ref to callback ref

[koenpunt]

since string refs are legacy

https://facebook.github.io/react/docs/refs-and-the-dom.html#legacy-api-string-refs

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

[mention-bot]

@koenpunt, thanks for your PR! By analyzing the history of the files in this pull request, we identified @jbaxleyiii to be a potential reviewer.

[jbaxleyiii]

@koenpunt thank you for this! I'm happy to merge it once the CI comes back green 👍

[koenpunt]

Oh well, that was Github's annoying auto-whitespace-insert in the online editor.
Should be all good now, also cleaned up the commits.