Run govuk-prototype-kit migration

This is the result of running `npx govuk-prototype-kit@latest migrate`.
This required a bit of preparation (see the previous commit), which
undid some of our customisations, and the migration itself removes some
of our customisation and Heroku config. This will be brought back in the
proceeding commit

.npmrc

@@ -1,2 +1 @@
 audit=false
-script-shell=bash

app/assets/javascripts/application.js

@@ -1,10 +1,8 @@
-/* global $ */
+//
+// For guidance on how to add JavaScript see:
+// https://prototype-kit.service.gov.uk/docs/adding-css-javascript-and-images
+//
 
-// Warn about using the kit in production
-if (window.console && window.console.info) {
-  window.console.info('GOV.UK Prototype Kit - do not use for production')
-}
-
-$(document).ready(function () {
-  window.GOVUKFrontend.initAll()
+window.GOVUKPrototypeKit.documentReady(() => {
+  // Add JavaScript here
 })