Sync with reactjs.org @ 6349ec18

[reactjs-translation-bot]

This PR was automatically generated.

Merge changes from reactjs.org at 6349ec18

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.

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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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