Align phase banner on service manual homepage

Add new `service_manual_homepage?` presenter method.

When on the service_manual_homepage, the phase banner will be wrapped in a div element with the `govuk-width-container` class to align it with the rest of the content on the page.

The service manual homepage layout is different from the rest of the service manual as it contains an app-hero banner that needs to span the full width of the page with a blue background, but we still want the phase banner to be contained.

app/presenters/content_item_presenter.rb

@@ -97,6 +97,10 @@ def service_manual?
     false
   end
 
+  def service_manual_homepage?
+    false
+  end
+
   def render_guide_as_single_page?
     # /how-to-vote
     content_id == "9315bc67-33e7-42e9-8dea-e022f56dabfa" && voting_is_open?

app/presenters/service_manual_presenter.rb

@@ -15,6 +15,10 @@ def service_manual?
     true
   end
 
+  def service_manual_homepage?
+    content_item["document_type"] == "service_manual_homepage"
+  end
+
   def show_phase_banner?
     false
   end

app/views/layouts/application.html.erb

@@ -10,7 +10,13 @@
       <%= render 'govuk_publishing_components/components/phase_banner', phase: @content_item.phase %>
     <% end %>
     <% if @content_item.service_manual? %>
+      <% if @content_item.service_manual_homepage? %>
+        <div class="govuk-width-container">
+          <%= render_phase_label @content_item, content_for(:phase_message) %>
+        </div>
+      <% else %>
         <%= render_phase_label @content_item, content_for(:phase_message) %>
+      <% end %>
     <% end %>
 
     <% if @content_item.show_default_breadcrumbs? %>