add postcss-preset-env, remove autoprefixer

Created by: heygrady

https://github.com/csstools/postcss-preset-env

There is now a postcss plugin that attempts to do for CSS what babel-preset-env does for js. It reads the same browserlist as babel-preset-env. This makes it possible to write advanced CSS and have it compiled for your target browsers.

This PR removes autoprefixer because postcss-preset-env includes it. The end result is the exact same output as before but with more functionality. The big difference is that you can now do something crazy like use css vars with fallbacks for older browsers. (Using CSS vars instead of using SCSS has some real world advantages, like the ability to manipulate those vars with JS.)

:root {
  --background: #282c34;
}

.App-header {
  background-color: var(--background);
}