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/client/autoIndex/App.js

@@ -1,6 +1,7 @@
+/** @jsx jsx */
 import React from 'react';
 import PropTypes from 'prop-types';
-import { Global, css } from '@emotion/core';
+import { Global, css, jsx } from '@emotion/core';
 
 import DirectoryListing from './DirectoryListing';
 

modules/client/autoIndex/DirectoryListing.js

@@ -1,5 +1,7 @@
+/** @jsx jsx */
 import React from 'react';
 import PropTypes from 'prop-types';
+import { jsx } from '@emotion/core';
 import formatBytes from 'pretty-bytes';
 import sortBy from 'sort-by';