Move ES6 components to their own file

This change is part of the process for upgrading apps to a new version of
govuk_publishing_components that uses GOV.UK frontend V5.1

This is one of the steps detailed in the [Upgrading to GOV.UK Frontend v5 document](https://docs.google.com/document/d/1uwip7pzQwM7t5ghn9_8KrsWXLePSRUltFPZ5fYw6Ab8/edit).

The purpose of this is to prevent browsers that don't support ES6 attempting to load ES6 Javascript from our components.

We know browser that don't support ES6 as grade X browsers, and these browsers also don't support
the `type = "module"` attribute in script tags. So we use this to prevent such code being loaded by ES6 Browsers.

The 'all_components' require as been removed to avoid grade X browsers calling ES6 javascript

app/assets/config/manifest.js

@@ -2,5 +2,6 @@
 //= link application.css
 //= link legacy_layout.css
 //= link application.js
+//= link es6-components.js
 //= link legacy_layout.js
 //= link html5.js

app/assets/javascripts/application.js

@@ -10,5 +10,4 @@
 // WARNING: THE FIRST BLANK LINE MARKS THE END OF WHAT'S TO BE PROCESSED, ANY BLANK LINE SHOULD
 // GO AFTER THE REQUIRES BELOW.
 //
-//= require govuk_publishing_components/dependencies
-//= require govuk_publishing_components/all_components
+//= require govuk_publishing_components/dependencies

app/assets/javascripts/es6-components.js

@@ -0,0 +1,15 @@
+// These modules from govuk_publishing_components
+// depend on govuk-frontend modules. govuk-frontend
+// now targets browsers that support `type="module"`.
+//
+// To gracefully prevent execution of these scripts
+// on browsers that don't support ES6, this script
+// should be included in a `type="module"` script tag
+// which will ensure they are never loaded.
+
+//= require govuk_publishing_components/components/button
+//= require govuk_publishing_components/components/checkboxes
+//= require govuk_publishing_components/components/error-summary
+//= require govuk_publishing_components/components/layout-header
+//= require govuk_publishing_components/components/radio
+//= require govuk_publishing_components/components/skip-link

app/views/layouts/application.html.erb

@@ -1,6 +1,7 @@
 <% content_for :head do %>
   <%= stylesheet_link_tag "legacy_layout", :media => "all" %>
   <%= javascript_include_tag "legacy_layout" %>
+  <%= javascript_include_tag "es6-components", type: "module" %>
   <%= csrf_meta_tag %>
   <%= render "layouts/google_tag_manager" %>
   <%= yield :extra_headers %>