[Cashtab] Upgrading pkg dependencies

Summary: Upgrading npm pkgs. Because react-app is ejected in Cashtab, also modifying webpackDevServer.config.js to correct type error; see https://stackoverflow.com/questions/60234640/typeerror-err-invalid-arg-type-the-path-argument-must-be-of-type-string-re

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9337