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).

modules/utils/getScripts.js

@@ -0,0 +1,16 @@
+import invariant from 'invariant';
+
+import getEntryPoint from './getEntryPoint';
+
+export default function getScripts(entryName, globalURLs) {
+  const entryPoint = getEntryPoint(entryName, 'iife');
+
+  invariant(entryPoint, 'Invalid entry name "%s"', entryName);
+
+  const globalScripts = entryPoint.globalImports.map(id => {
+    invariant(globalURLs[id], 'Missing global URL for id "%s"', id);
+    return globalURLs[id];
+  });
+
+  return globalScripts.concat(entryPoint.url);
+}