[Cashtab] Improve scrolling and header minification

Summary:
This still has not been quite right. Improve implementation so that it is actually getting expected behavior (i.e., header is minified when user scrolls past 63 px, header is not un-minified until user scrolls back above 15px)

Note: react router not resetting scroll to 0 on nav is expected behavior. It can be fixed with this, but we need to migrate the routers first. https://reactrouter.com/en/main/components/scroll-restoration

Note: I tried to add integration tests for the behavior, however react testing library uses JSDOM which, apparently, does not know anything about scrolling. We would need to use a browser-based testing library. We may need to do this eventually anyway to properly test the browser extension. But we can't do it today.

Test Plan: npm test, npm start and better behavior on receive and send screens (screens that do not usually have any scrolling).

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16035