FIX: Polyfill Promise for IE11 (#9057)

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

plugins/discourse-internet-explorer/public/js/ie.js

@@ -970,3 +970,12 @@ if (!String.prototype.startsWith) {
   return ES6;
 });
 /* eslint-enable */
+
+// Polyfill Promise - used by popper.js
+window.addEventListener(
+  "load",
+  function() {
+    window.Promise = require("rsvp").Promise;
+  },
+  false
+);