Replace use of Array.from

Array.from isn't supported by any version of IE.

We still need to convert a nodelist to an array so
we can either:
1. include the polyfill for Array.from from core-js
2. use [].slice

We don't use Array.from anywhere else so this uses
the second one.

app/assets/javascripts/apiKey.js

@@ -29,9 +29,10 @@
 
     this.getRangeFromElement = function (keyElement) {
       const range = document.createRange();
+      const childNodes = Array.prototype.slice.call(keyElement.childNodes);
       let prefixIndex = -1;
 
-      Array.from(keyElement.childNodes).forEach((el, idx) => {
+      childNodes.forEach((el, idx) => {
         if ((el.nodeType === 1) && el.classList.contains('govuk-visually-hidden')) {
           prefixIndex = idx;
         }