Mount /component-guide

This should be enabled on every app which pulls in govuk_publishing_components.

Similarly, moves the (deprecated) govuk_admin_template `/style-guide` route out of the dev block so that it's available on production. It was decided in https://trello.com/c/ITcz5b1G/3417-enable-component-guide-on-non-dev-environments that there's no reason we need to lock this down to dev env only.

config/routes.rb

@@ -17,7 +17,6 @@
     GovukHealthcheck::SidekiqRedis,
   )
 
-  if Rails.env.development?
-    get "/styleguide" => "govuk_admin_template/style_guide#index"
-  end
+  mount GovukPublishingComponents::Engine, at: "/component-guide"
+  get "/styleguide" => "govuk_admin_template/style_guide#index"
 end