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/README.md

@@ -0,0 +1,48 @@
+# @emotion/core
+
+> Simple styling in React.
+
+## Install
+
+```bash
+yarn add @emotion/core
+```
+
+## Usage
+
+```jsx
+/** @jsx jsx */
+import { jsx, css, Global, ClassNames } from '@emotion/core'
+
+render(
+  <div css={{ color: 'hotpink' }}>
+    <div
+      css={css`
+        color: green;
+      `}
+    />
+    <Global
+      styles={{
+        body: {
+          margin: 0,
+          padding: 0
+        }
+      }}
+    />
+    <ClassNames>
+      {({ css, cx }) => (
+        <div
+          className={cx(
+            'some-class',
+            css`
+              color: yellow;
+            `
+          )}
+        />
+      )}
+    </ClassNames>
+  </div>
+)
+```
+
+More documentation is available at https://emotion.sh.