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/react-dom@16.7.0/build-info.json

@@ -0,0 +1,8 @@
+{
+  "branch": "master",
+  "buildNumber": "12928",
+  "checksum": "7d45248",
+  "commit": "4a1072194",
+  "environment": "ci",
+  "reactVersion": "16.6.1-canary-4a1072194"
+}