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';
 

package.json

@@ -32,7 +32,8 @@
     "react_ujs": "^2.6.0",
     "select2": "^4.0.11",
     "trix": "^1.2.1",
-    "turbolinks": "^5.2.0"
+    "turbolinks": "^5.2.0",
+    "whatwg-fetch": "^3.0.0"
   },
   "devDependencies": {
     "babel-eslint": "^10.0.3",

yarn.lock

@@ -8931,6 +8931,11 @@ websocket-extensions@>=0.1.1:
   resolved "https://registry.yarnpkg.com/websocket-extensions/-/websocket-extensions-0.1.3.tgz#5d2ff22977003ec687a4b87073dfbbac146ccf29"
   integrity sha512-nqHUnMXmBzT0w570r2JpJxfiSD1IzoI+HGVdd3aZ0yNi3ngvQ4jv1dtHt5VGxfI2yj5yqImPhOK4vmIh2xMbGg==
 
+whatwg-fetch@^3.0.0:
+  version "3.0.0"
+  resolved "https://registry.yarnpkg.com/whatwg-fetch/-/whatwg-fetch-3.0.0.tgz#fc804e458cc460009b1a2b966bc8817d2578aefb"
+  integrity sha512-9GSJUgz1D4MfyKU7KRqwOjXCXTqWdFNvEr7eUBYchQiVc744mqK/MzXPNR2WsPkmkOa4ywfg8C2n8h+13Bey1Q==
+
 which-module@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/which-module/-/which-module-1.0.0.tgz#bba63ca861948994ff307736089e3b96026c2a4f"