[Cashtab] Better button sizing, spacing, and hover

Summary:
T2650

Make button take up all alotted space, so you do not have to click just on the icon. Add hover effect for non-mobile devices.

Ref https://stackoverflow.com/questions/17233804/how-to-prevent-sticky-hover-effects-for-buttons-on-touch-devices for why we need the media query (otherwise if you push a button, it gets the hover state on mobile)

Can verify mobile at https://cashtab-local-dev.netlify.app/

Test Plan: npm test, npm start and test hover effect

Reviewers: #bitcoin_abc, Klakurka

Reviewed By: #bitcoin_abc, Klakurka

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D15157