Add IntersectionObserver polyfill for Safari fixes #1769

package.json

@@ -34,6 +34,7 @@
     "classnames": "^2.2.6",
     "css-ns": "^1.2.2",
     "deepmerge": "^3.1.0",
+    "intersection-observer": "^0.5.1",
     "manifesto.js": "^3.0.9",
     "node-fetch": "2.1.1",
     "node-sass": "^4.9.2",

src/components/CanvasThumbnail.js

 import React, { Component } from 'react';
 import PropTypes from 'prop-types';
+import 'intersection-observer'; // polyfill needed for Safari
 import IntersectionObserver from '@researchgate/react-intersection-observer';
 
 /**