Stop using isomorphic-unfetch

fetch is built into the browser

src/state/sagas/iiif.js

 import {
   all, call, put, select, takeEvery,
 } from 'redux-saga/effects';
-import fetch from 'isomorphic-unfetch';
 import { Utils } from 'manifesto.js';
 import normalizeUrl from 'normalize-url';
 import ActionTypes from '../actions/action-types';