Fix repetitive database queries from #30040 (#30259)

app/presenters/instance_presenter.rb

@@ -81,4 +81,16 @@ class InstancePresenter < ActiveModelSerializers::Model
   def mascot
     @mascot ||= Rails.cache.fetch('site_uploads/mascot') { SiteUpload.find_by(var: 'mascot') }
   end
+
+  def favicon
+    return @favicon if defined?(@favicon)
+
+    @favicon ||= Rails.cache.fetch('site_uploads/favicon') { SiteUpload.find_by(var: 'favicon') }
+  end
+
+  def app_icon
+    return @app_icon if defined?(@app_icon)
+
+    @app_icon ||= Rails.cache.fetch('site_uploads/app_icon') { SiteUpload.find_by(var: 'app_icon') }
+  end
 end