Move banner into partial

This ensures that we can deploy most of the functionality in advance and switch over the banner content in the final deploy.

As we want to be able to deploy just the content, the tests need to support
a partial both with and without content. To do this, we override the view path to stub templates
replacement. This is borrowed from rspec-rails’ `stub_template` method
https://github.com/rspec/rspec-rails/blob/e8054a1cd03044f725030fe8315952
cf3799a395/lib/rspec/rails/example/view_example_group.rb#L86-L88

app/helpers/promo_banner_helper.rb

@@ -0,0 +1,5 @@
+module PromoBannerHelper
+  def render_promo_banner
+    render partial: "promo_banner"
+  end
+end

app/views/homepage/_promo_banner_example.html.erb

@@ -0,0 +1,6 @@
+<div id="homepage-promo-banner">
+  <div class="banner-message">
+    <p><strong>Some Title</strong> On some date something has happened.
+    <a href="https://something.gov.uk" rel="external nofollow">More&nbsp;information<span class="visuallyhidden"> about things</span></a></p>
+  </div>
+</div>

app/views/homepage/index.html.erb

@@ -5,7 +5,7 @@
 <% content_for :title, "Welcome to GOV.UK" %>
 <% content_for :body_classes, "homepage" %>
 
-<%= render :partial => "campaign_notification" %>
+<%= campaign_notification_content = render partial: "campaign_notification" %>
 <main id="content" class="root-index" role="main">
   <header class="homepage-top">
     <div class="homepage-top-inner">
@@ -46,12 +46,7 @@
     </div>
   </header>
 
-  <div id="homepage-promo-banner">
-    <div class="banner-message">
-      <p><strong>Some Title</strong> On [some date] [something will happen / has happened].
-      <a href="https://www.gov.uk" rel="external nofollow">More&nbsp;information</a></p>
-    </div>
-  </div>
+  <%= render_promo_banner if campaign_notification_content.strip.blank? %>
 
   <div id="homepage" class="homepage-content">
     <div class="homepage-content-inner">

test/functional/homepage_controller_test.rb

@@ -11,5 +11,38 @@ class HomepageControllerTest < ActionController::TestCase
       get :index
       assert_equal "max-age=1800, public", response.headers["Cache-Control"]
     end
+
+    context "with a blank promo banner partial" do
+      should "not render the promo banner" do
+        stub_template "homepage/_promo_banner.html.erb" => "<%# foo %>"
+
+        get :index
+        assert_template "homepage/_promo_banner"
+        refute @response.body.include?("homepage-promo-banner")
+      end
+    end
+
+    context "with a promo banner partial" do
+      should "render promo banner" do
+        stub_template "homepage/_promo_banner.html.erb" => <<-EOF
+        <div id="homepage-promo-banner">
+          <div class="banner-message">
+            <p><strong>Some Title</strong> On some date something has happened.
+            <a href="https://something.gov.uk" rel="external nofollow">More&nbsp;information</a></p>
+          </div>
+        </div>
+        EOF
+
+        get :index
+        assert_template "homepage/_promo_banner"
+        assert @response.body.include?("homepage-promo-banner")
+        assert @response.body.include?("Some Title")
+      end
+    end
+  end
+
+  def stub_template(hash)
+    require 'action_view/testing/resolvers'
+    @controller.view_paths.unshift(ActionView::FixtureResolver.new(hash))
   end
 end