frontend: Remove lib/cluster module

This module was changed and now just exports a constant. That can be
done in the lib/util module instead.

frontend/src/components/common/ActionsNotifier.js

@@ -4,7 +4,7 @@ import { useSnackbar } from 'notistack';
 import React from 'react';
 import { useDispatch, useSelector } from 'react-redux';
 import { useHistory } from 'react-router-dom';
-import { CLUSTER_ACTION_GRACE_PERIOD } from '../../lib/cluster';
+import { CLUSTER_ACTION_GRACE_PERIOD } from '../../lib/util';
 
 export default function ActionsNotifier() {
   const { enqueueSnackbar, closeSnackbar } = useSnackbar();

frontend/src/lib/cluster.js

-export const CLUSTER_ACTION_GRACE_PERIOD = 5000; // ms

frontend/src/lib/util.js

@@ -7,6 +7,8 @@ TimeAgo.addLocale(en);
 
 const TIME_AGO = new TimeAgo();
 
+export const CLUSTER_ACTION_GRACE_PERIOD = 5000; // ms
+
 export function timeAgo(date) {
   return TIME_AGO.format(new Date(date), 'time');
 }

frontend/src/redux/sagas/sagas.js

 import { all, call, cancelled, delay, put, race, take, takeEvery } from 'redux-saga/effects';
-import { CLUSTER_ACTION_GRACE_PERIOD } from '../../lib/cluster';
+import { CLUSTER_ACTION_GRACE_PERIOD } from '../../lib/util';
 import { CLUSTER_ACTION, CLUSTER_ACTION_CANCEL, updateClusterAction } from '../actions/actions';
 
 function newActionKey() {