site: fix intersection-observer polyfill

package.json

@@ -125,6 +125,7 @@
     "fetch-jsonp": "^1.0.3",
     "glob": "^7.1.1",
     "immutability-helper": "^2.5.0",
+    "intersection-observer": "^0.5.0",
     "jest": "^22.4.2",
     "jsonml.js": "^0.1.0",
     "lint-staged": "^7.0.0",

site/theme/template/Content/MainContent.jsx

@@ -85,8 +85,8 @@ export default class MainContent extends React.Component {
     if (this.scroller) {
       this.scroller.disable();
     }
-    // eslint-disable-next-line
-    const scrollama = require('scrollama');
+    require('intersection-observer'); // eslint-disable-line
+    const scrollama = require('scrollama'); // eslint-disable-line
     this.scroller = scrollama();
     this.scroller
       .setup({