[Cashtab] Add commas to formatted displays of large numbers

Summary: Add new formatBalance function to cashMethods.js to add commas to displayed values of large numbers. Add unit tests. Implement for tokens as well.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9283