Create new assets for transition to design system

Rename `application.js` and `application.scss` to
`legacy-application.scss` and `legacy-application.js` in
preparation to use the `layout_for_admin` component from
the govuk_publishing_components gem. Update the configuration
manifest and point `application.html.erb` to the new legacy
asset files.

app/assets/config/manifest.js

@@ -1,3 +1,6 @@
 //= link_tree ../images
 //= link application.js
 //= link application.css
+
+//= link legacy-application.js
+//= link legacy-application.css

app/assets/javascripts/application.js

@@ -1,23 +1,2 @@
-//= require select2
-//= require jquery-ui/widgets/sortable
-//= require moment
-//= require mousetrap
-//= require jquery-ui.custom.min
-//= require_directory ./vendor
-//= require_directory .
-//= require_directory ./modules
-//= require jquery_nested_form
-
-// System wide behaviours
-$(function () {
-  $('a.preview').attr('target', '_blank')
-  $('form.preview').attr('target', '_blank')
-  $('.select2').select2({ allowClear: true })
-})
-
-// System wide library functions
-var GovUKGuideUtils = { // eslint-disable-line no-unused-vars
-  convertToSlug: function (title) {
-    return title.toLowerCase().replace(/[^\w ]+/g, '').replace(/ +/g, '-')
-  }
-}
+//= require govuk_publishing_components/dependencies
+//= require govuk_publishing_components/all_components

app/assets/javascripts/legacy-application.js

@@ -0,0 +1,23 @@
+//= require select2
+//= require jquery-ui/widgets/sortable
+//= require moment
+//= require mousetrap
+//= require jquery-ui.custom.min
+//= require_directory ./vendor
+//= require_directory .
+//= require_directory ./modules
+//= require jquery_nested_form
+
+// System wide behaviours
+$(function () {
+  $('a.preview').attr('target', '_blank')
+  $('form.preview').attr('target', '_blank')
+  $('.select2').select2({ allowClear: true })
+})
+
+// System wide library functions
+var GovUKGuideUtils = { // eslint-disable-line no-unused-vars
+  convertToSlug: function (title) {
+    return title.toLowerCase().replace(/[^\w ]+/g, '').replace(/ +/g, '-')
+  }
+}

app/assets/stylesheets/application.scss

@@ -1,25 +1,2 @@
-@import "govuk_admin_template";
-@import "select2";
-@import "./vendor/select2-bootstrap";
-@import "colors";
-@import "bootstrap_and_overrides";
-@import "forms";
-
-// Components
-@import "sidebar_nav";
-@import "sortable_accordion";
-@import "sortable_table";
-@import "panel_part";
-@import "diff";
-@import "notes";
-@import "workflow";
-@import "broken_links_report";
-@import "error_summary";
-
-// Pages
-@import "downtime";
-@import "smart_answer_builder";
-@import "smart_answer_flowchart";
-
 // GOVUK Design System
 @import "govuk_publishing_components/all_components";

app/assets/stylesheets/legacy-application.scss

@@ -0,0 +1,25 @@
+@import "govuk_admin_template";
+@import "select2";
+@import "./vendor/select2-bootstrap";
+@import "colors";
+@import "bootstrap_and_overrides";
+@import "forms";
+
+// Components
+@import "sidebar_nav";
+@import "sortable_accordion";
+@import "sortable_table";
+@import "panel_part";
+@import "diff";
+@import "notes";
+@import "workflow";
+@import "broken_links_report";
+@import "error_summary";
+
+// Pages
+@import "downtime";
+@import "smart_answer_builder";
+@import "smart_answer_flowchart";
+
+// GOVUK Design System
+@import "govuk_publishing_components/all_components";

app/views/layouts/application.html.erb

@@ -5,7 +5,7 @@
   <%= favicon_link_tag "govuk_admin_template/#{favicon}" %>
 <% end %>
 <% content_for :head do %>
-  <%= stylesheet_link_tag "application", :media => "all" %>
+  <%= stylesheet_link_tag "legacy-application", :media => "all" %>
   <%= csrf_meta_tag %>
   <%= yield :extra_headers %>
 <% end %>
@@ -54,7 +54,7 @@
 <% content_for :footer_version, ENV.fetch("SENTRY_RELEASE", "null")[0..18] %>
 
 <% content_for :body_end do %>
-  <%= javascript_include_tag "application" %>
+  <%= javascript_include_tag "legacy-application" %>
   <%= yield :extra_javascript %>
 <% end %>