Change css library to emotion

[danr]

New times, new css lib. https://emotion.sh/docs/introduction

This PR mainly changes from the material ui's makeStyles to emotion.css + a bunch of vaguelly related clean-ups.

Benefits are:

• can write css as string (though not used in this PR)
• react css prop (though not used in this PR)
• sourcemaps
• sensible snapshot testing
• no need for hook
• dynamic properties using @emotion/core.ClassNames
• automatic labels https://github.com/emotion-js/emotion/tree/master/packages/babel-plugin-emotion