Sync with reactjs.org @ 4fc709d0

[reactjs-translation-bot]

This PR was automatically generated.

Merge changes from reactjs.org at 4fc709d0

The following files have conflicts and may need new translations:

• [ ] content/tutorial/tutorial.md
• [ ] content/docs/uncontrolled-components.md
• [ ] content/docs/typechecking-with-proptypes.md
• [ ] content/docs/testing-recipes.md
• [ ] content/docs/testing-environments.md
• [ ] content/docs/strict-mode.md
• [ ] content/docs/refs-and-the-dom.md
• [ ] content/docs/reference-react-component.md
• [ ] content/docs/reference-javascript-environment-requirements.md
• [ ] content/docs/reference-events.md
• [ ] content/docs/reference-dom-elements.md
• [ ] content/docs/optimizing-performance.md
• [ ] content/docs/introducing-jsx.md
• [ ] content/docs/higher-order-components.md
• [ ] content/docs/getting-started.md
• [ ] content/docs/faq-versioning.md
• [ ] content/docs/faq-ajax.md
• [ ] content/docs/context.md
• [ ] content/docs/code-splitting.md
• [ ] content/docs/cdn-links.md
• [ ] content/docs/add-react-to-a-website.md

Please fix the conflicts by pushing new commits to this pull request, either by editing the files directly on GitHub or by checking out this branch.

DO NOT SQUASH MERGE THIS PULL REQUEST!

Doing so will "erase" the commits from master and cause them to show up as conflicts the next time we merge.

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