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/components/shared/banner_component.rb

@@ -1,8 +1,16 @@
 class Shared::BannerComponent < ApplicationComponent
-  attr_reader :banner
+  attr_reader :banner_or_section
 
-  def initialize(banner)
-    @banner = banner
+  def initialize(banner_or_section)
+    @banner_or_section = banner_or_section
+  end
+
+  def banner
+    @banner ||= if banner_or_section.respond_to?(:sections)
+                  banner_or_section
+                else
+                  Banner.in_section(banner_or_section).with_active.sample
+                end
   end
 
   private