[Cashtab] Webpack upgrade part 3, polyfill support

Summary:
T1682

This diff implements step 3 outlined in the task description. These polyfill fixes are necessary for bchjs functions used in Cashtab, as the most recent version of create react app does not support polyfills by default.

Test Plan:
cd web/cashtab-v2
npm i
npm start and observe default react app
npm run build and build success

Reviewers: kieran709, #bitcoin_abc

Reviewed By: kieran709, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11152