Move Javascript modules to es6-components

To prevent browsers evaluating JS that isn't supported, components that
contain code from govuk-frontend have been moved to a separate file
`es6-components.js` which is included in `application.html.erb` as a
script tag with `type="module"`.

app/assets/config/manifest.js

@@ -1,5 +1,6 @@
 //= link_tree ../images
 //= link application.js
+//= link es6-components.js
 //= link webchat.js
 //= link application.css
 //= link components/_back-to-top.css

app/assets/javascripts/application.js

@@ -1,13 +1,10 @@
 //= require govuk_publishing_components/lib
-//= require govuk_publishing_components/components/accordion
 //= require govuk_publishing_components/components/details
-//= require govuk_publishing_components/components/error-summary
 //= require govuk_publishing_components/components/govspeak
 //= require govuk_publishing_components/components/image-card
 //= require govuk_publishing_components/components/intervention
 //= require govuk_publishing_components/components/metadata
 //= require govuk_publishing_components/components/print-link
-//= require govuk_publishing_components/components/radio
 //= require govuk_publishing_components/components/single-page-notification-button
 //= require govuk_publishing_components/components/step-by-step-nav
 

app/assets/javascripts/es6-components.js

@@ -0,0 +1,3 @@
+//= require govuk_publishing_components/components/accordion
+//= require govuk_publishing_components/components/error-summary
+//= require govuk_publishing_components/components/radio

app/views/layouts/application.html.erb

@@ -53,6 +53,7 @@
   <% end %>
 
   <%= stylesheet_link_tag "application", :media => "all", integrity: false %>
+  <%= javascript_include_tag 'es6-components', type: "module" %>
   <%= javascript_include_tag "application", integrity: false %>
   <%= csrf_meta_tags %>
   <%= render 'govuk_publishing_components/components/meta_tags', content_item: @content_item.content_item %>