javascript: add a window.fetch polyfill

This allows the autosave to work on browsers that don't support fetch:

- IE 11
- Edge < 14
- Safari < 11

app/javascript/packs/application.js

@@ -3,6 +3,7 @@ import Turbolinks from 'turbolinks';
 import Rails from '@rails/ujs';
 import * as ActiveStorage from '@rails/activestorage';
 import '@rails/actiontext';
+import 'whatwg-fetch'; // window.fetch polyfill
 import Chartkick from 'chartkick';
 import Highcharts from 'highcharts';