Simplify rendering a banner

Now the banner component accepts either a banner or a section and loads
the banner if it's a section, so we don't have to add the `@banners`
variable in several controllers.

app/controllers/pages_controller.rb

@@ -6,7 +6,6 @@ class PagesController < ApplicationController
 
   def show
     @custom_page = SiteCustomization::Page.published.find_by(slug: params[:id])
-    @banners = Banner.in_section("help_page").with_active
 
     if @custom_page.present?
       @cards = @custom_page.cards