[Cashtab] Show cumulative depth bars on OrderBook to match normal exchange UX

Summary: The current setup is confusing. Make depth bars cumulative so it looks like any other exchange OrderBook.

Test Plan:
npm test, check screenshots.

can also check the dev site at https://cashtab-local-dev.netlify.app/ (make sure to hard refresh)

I considered adding this to integration tests but I think the impact of confirming css widths is detrimental. The calc here is not particularly complicated and it's immediately obvious if stuff is loading improperly.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17434