Make use of JS Modules less confusing

It's not obvious how the code that includes JS
Modules in the frontend build works.

This adds lots of comments to explain the various
bits and flattens `modules/all.mjs` to just be a
single function that starts off the window.GOVUK
namespace.

Also removes `module/all.js` from the repo'. It's
an artefact used by the frontend build so
shouldn't be included as source code.

app/assets/javascripts/modules/all.mjs

@@ -1,5 +1,15 @@
+// JS Module used to combine all the JS modules used in the application into a single entry point,
+// a bit like `app/__init__` in the Flask app.
+//
+// When processed by a bundler, this is turned into a Immediately Invoked Function Expression (IIFE)
+// and saved as `all.js` in the same folder as this file. The IIFE format allows it to run in
+// browsers that don't support JS Modules.
+//
+// Exported items will be added to the window.GOVUK namespace.
 import Header from 'govuk-frontend/components/header/header';
 
+// Copy of the initAll function from https://github.com/alphagov/govuk-frontend/blob/v2.13.0/src/all.js
+// except it only includes, and initialises, the components used by this application.
 function initAll (options) {
   // Set the options to an empty object by default if no options are passed.
   options = typeof options !== 'undefined' ? options : {}
@@ -13,7 +23,12 @@ function initAll (options) {
   new Header($toggleButton).init()
 }
 
-export {
-  Header,
-  initAll
+// Create separate namespace for GOVUK Frontend.
+var Frontend = {
+  "Header": Header,
+  "initAll": initAll
+}
+
+export {
+  Frontend
 }