[Cashtab] Webpack upgrade p10, first working version

Summary:
T1682

Combines steps 10 and 11, since step 11 is implementing the functionality that is removed in step 10 (as it no longer works in this version of webpack, since less loader is gone).

Test Plan:

npm ci
npm start

Dismiss lint errors

App should load (though with visible css issues, to be patched in coming steps)

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11250