React 17 support

`--legacy-peer-deps` seems to be fine for now..

```
npm ERR! Found: react@17.0.2
npm ERR! node_modules/react
npm ERR!   react@"^17.0.2" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peer react@"^16.8.0" from react-netlify-forms@1.3.1
npm ERR! node_modules/react-netlify-forms
npm ERR!   react-netlify-forms@"^1.3.1" from the root project
```