Migrate to plain CSS (remove CSS modules) (#659)

* Migrate to plain CSS (remove CSS modules)

Change `prefixer` to a function instead of a proxy

* Switch prefix to `nc`

* Replace prefixer with literal class names

* Remove prefixer

* Fix migration errors

* fix compose migrations

* Remove unnecessary theme imports

* Remove old CSS import

* fix sticky toolbar positioning

* update to cssnano v4 so preset is used

* fix css pseudo selectors

* update lockfile

package.json

@@ -90,6 +90,7 @@
     "caniuse-lite": "^1.0.30000745",
     "cross-env": "^5.0.2",
     "css-loader": "^0.28.7",
+    "cssnano": "^v4.0.0-rc.2",
     "deep-equal": "^1.0.1",
     "enzyme": "^3.0.0",
     "enzyme-adapter-react-15": "^1.0.0",