Add fetch polyfill

Fetch has been broken for a while in Internet Explorer 11. This adds a
tiny fetch polyfill (500 bytes) in order to mitigate the issue, until we
are officially drop Internet Explorer 11 support later this year.

- https://gitlab.com/gitlab-org/gitlab/issues/25700
- https://gitlab.com/gitlab-org/gitlab/issues/43662

app/assets/javascripts/commons/polyfills.js

 // Browser polyfills
+
+/**
+ * Polyfill: fetch
+ * @what https://fetch.spec.whatwg.org/
+ * @why Because Apollo GraphQL client relies on fetch
+ * @browsers Internet Explorer 11
+ * @see https://caniuse.com/#feat=fetch
+ */
+import 'unfetch/polyfill/index';
 import 'formdata-polyfill';
 import './polyfills/custom_event';
 import './polyfills/element';

changelogs/unreleased/leipert-polyfills-improvements.yml

+---
+title: Polyfill fetch for Internet Explorer 11
+merge_request: 26366
+author:
+type: fixed

package.json

@@ -125,6 +125,7 @@
     "tiptap-commands": "^1.4.0",
     "tiptap-extensions": "^1.8.0",
     "underscore": "^1.9.0",
+    "unfetch": "^4.1.0",
     "url-loader": "^2.1.0",
     "visibilityjs": "^1.2.4",
     "vue": "^2.6.10",

yarn.lock

@@ -11453,6 +11453,11 @@ underscore@~1.8.3:
   resolved "https://registry.yarnpkg.com/underscore/-/underscore-1.8.3.tgz#4f3fb53b106e6097fcf9cb4109f2a5e9bdfa5022"
   integrity sha1-Tz+1OxBuYJf8+ctBCfKl6b36UCI=
 
+unfetch@^4.1.0:
+  version "4.1.0"
+  resolved "https://registry.yarnpkg.com/unfetch/-/unfetch-4.1.0.tgz#6ec2dd0de887e58a4dee83a050ded80ffc4137db"
+  integrity sha512-crP/n3eAPUJxZXM9T80/yv0YhkTEx2K1D3h7D1AJM6fzsWZrxdyRuLN0JH/dkZh1LNH8LxCnBzoPFCPbb2iGpg==
+
 unherit@^1.0.4:
   version "1.1.1"
   resolved "https://registry.yarnpkg.com/unherit/-/unherit-1.1.1.tgz#132748da3e88eab767e08fabfbb89c5e9d28628c"