[Cashtab] Minify and pin wallet header on scroll

Summary:
T2207

Show a minified version of the wallet header when user scrolls down. Include CSS improvements for alignment of items in header.

Test Plan:
npm test, npm start and test scroll behavior and appearance.

npm run extension and ok there too

This diff is live at https://cashtab-local-dev.netlify.app for testing on mobile

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15947