Support React 17

[thien-do]

React 17 release notes: https://reactjs.org/blog/2020/10/20/react-v17.html (not many breaking changes)

Related code:

firebaseui-web-react/package.json

Line 32 in 18862d2

"react": ">=15 <17",

[HugoGresse]

See #122
(already released in canary)