[Cashtab] Receive screen improvements

Summary:
QR code is an important part of any crypto wallet app. It's the only thing we want to see on the Receive screen and should take up most of the space.

Users do not read the entire address string. So, only show a preview.

Show the entire address in monospace blocks when it is copied.

Test Plan:
npm start, confirm the QR code can be scanned at widths > 500px and and 320px, confirm this looks better.
npm run extension or set dimensions at 400 width 600 height in dev console, confirm qr code renders above the fold
review added test cases and npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15304