Unremove react-app-polyfill which seems to be necessary for tests

(cherry picked from commit 4e744d9)

Author: jsharkey13

package.json

@@ -69,6 +69,7 @@
     "postcss-safe-parser": "4.0.1",
     "query-string": "^6.5.0",
     "react": "16.8.4",
+    "react-app-polyfill": "^0.2.2",
     "react-dev-utils": "^8.0.0",
     "react-dom": "16.8.4",
     "react-ga": "^2.5.7",