Add cookie banner

Once wired up, this will allow users to set their cookie preferences,
initially just for Google Tag Manager tracking

Cookie banners often link to a cookies page, but I've omitted that here
to keep the change focused. It might be good to add an option to include
this in future

Author: yndajas

lib/assets/stylesheets/_govuk_tech_docs.scss

@@ -17,6 +17,7 @@ $govuk-new-link-styles: true;
 @import "govuk/core/index";
 @import "govuk/objects/index";
 
+@import "govuk/components/cookie-banner/index";
 @import "govuk/components/footer/index";
 @import "govuk/components/header/index";
 @import "govuk/components/inset-text/index";

lib/source/layouts/core.erb

@@ -52,6 +52,28 @@
     <script>document.body.className += ' js-enabled' + ('noModule' in HTMLScriptElement.prototype ? ' govuk-frontend-supported' : '');</script>
 
     <div class="app-pane">
+      <div class="govuk-cookie-banner" data-nosnippet role="region" aria-label="Cookies on <%= config[:tech_docs][:service_name] %>">
+        <div class="govuk-cookie-banner__message govuk-width-container">
+          <div class="govuk-grid-row">
+            <div class="govuk-grid-column-two-thirds">
+              <h2 class="govuk-cookie-banner__heading govuk-heading-m">
+                Cookies on <%= config[:tech_docs][:service_name] %>
+              </h2>
+              <div class="govuk-cookie-banner__content">
+                <p class="govuk-body">We’d like to use analytics cookies so we can understand how you use the service and make improvements.</p>
+              </div>
+            </div>
+          </div>
+          <div class="govuk-button-group">
+            <button type="button" class="govuk-button" data-module="govuk-button">
+              Accept analytics cookies
+            </button>
+            <button type="button" class="govuk-button" data-module="govuk-button">
+              Reject analytics cookies
+            </button>
+          </div>
+        </div>
+      </div>
       <div class="app-pane__header toc-open-disabled">
         <a href="#content" class="govuk-skip-link" data-module="govuk-skip-link">Skip to main content</a>