Bug 1503876 Comment 5 Edit History

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Suggested steps to solve: 
1. Convert `TopSiteForm.jsx` into a functional React componet 
2. Implement the same [focus trap method](https://medium.com/cstech/achieving-focus-trapping-in-a-react-modal-component-3f28f596f35b) that was [implemented on `TopicSelection.jsx`](https://searchfox.org/mozilla-central/source/browser/components/newtab/content-src/components/DiscoveryStreamComponents/TopicSelection/TopicSelection.jsx#134-181).

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