Replace babel-polyfill with core-js and regenerator-runtime. (#176)

App not workinng in IE 11 with babel-polyfill.

package.json

@@ -44,7 +44,7 @@
     "@fortawesome/free-regular-svg-icons": "5.15.1",
     "@fortawesome/free-solid-svg-icons": "5.15.1",
     "@fortawesome/react-fontawesome": "0.1.13",
-    "babel-polyfill": "6.26.0",
+    "core-js": "^3.9.1",
     "classnames": "^2.2.6",
     "extract-react-intl-messages": "^4.1.1",
     "form-urlencoded": "^4.2.1",
@@ -65,7 +65,8 @@
     "redux-saga": "^1.1.3",
     "redux-thunk": "^2.3.0",
     "reselect": "^4.0.0",
-    "react-helmet": "^6.1.0"
+    "react-helmet": "^6.1.0",
+    "regenerator-runtime": "^0.13.7"
   },
   "devDependencies": {
     "@edx/frontend-build": "5.6.8",