Load React + Emotion from CDN URLs, use globals

Remove @emotion/babel-preset-css-prop and import { jsx } from
@emotion/core directly instead so Rollup can preserve imports order when
bundling (instead of @emotion/core automatically inserting itself as the
first import).

public/@emotion/core@10.0.6/dist/core.cjs.js

@@ -0,0 +1,7 @@
+'use strict';
+
+if (process.env.NODE_ENV === "production") {
+  module.exports = require("./core.cjs.prod.js");
+} else {
+  module.exports = require("./core.cjs.dev.js");
+}