Include Debugbar on development environments

See https://debugbar.dev/

Screenshots of this in action:

- ![Bar included at bottom of screen](https://github.com/user-attachments/assets/aea1f59f-cf7e-4e21-9210-f8e4d5b3a54a)
- ![Expanded bar, showing SQL queries](https://github.com/user-attachments/assets/0bd3d754-76e4-4075-aaa4-c2b89553f0b8)
- ![Requests are tracked](https://github.com/user-attachments/assets/34099245-d658-4cc1-83db-7e6d1c83ca69)

There is an option for using websockets for a faster implementation
(the polling approach takes a second to update). But this didn't
seem to work out of the box for me, unlike the polling approach
documented here:
https://debugbar.dev/docs/polling-mode/

Author: ChrisBAshton

app/views/layouts/design_system.html.erb

@@ -2,6 +2,7 @@
 
 <% if user_signed_in? %>
   <% content_for :head do %>
+    <%= debugbar_head  if defined? Debugbar %>
     <meta name="govuk:components_gem_version" content="<%= GovukPublishingComponents::VERSION %>">
     <%= javascript_include_tag "admin/domain-config" %>
     <%= javascript_include_tag "govuk_publishing_components/load-analytics" %>
@@ -99,4 +100,5 @@
       },
     ],
   } %>
+  <%= debugbar_body(mode: :poll, poll: { url: "http://whitehall-admin.dev.gov.uk", interval: 1000 }) if defined? Debugbar %>
 <% end %>

config/initializers/debug_bar.rb

@@ -0,0 +1,5 @@
+return unless (defined? Debugbar) && (Rails.env.development?)
+
+Debugbar.configure do |config|
+  config.enabled = true
+end