[Cashtab] Update dependencies

Summary:
Have had intermittent issues with npm start

Diff made by

rm -rf node_modules
rm package-lock.json
npm i

This resolves dependency alerts and gives clean output from npm audit. However, we get a new polyfill issue from a dependency of crypto-browserify. Resolve this with

https://www.npmjs.com/package/node-polyfill-webpack-plugin

Updated https://www.npmjs.com/package/@testing-library/react to latest version to resolve mysterious test failures caused by dep upgrades.

Test Plan:
npm test
npm start and app runs in dev mode
npm audit and found 0 vulnerabilities

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16900