[Cashtab] Reduce bundle size by about ~50%

Summary: By enabling the minimize option on webpack config, the bundled size of the largest chunk (~600KB) shrinks down to ~300KB.

Test Plan: npm build, check bundle size, test locally using serve (npm install -g serve): serve -s build.

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9568