Create Settings::BaseController (#9507)

Define `Settings::BaseController#set_body_classes` so that sub classes
inherit `Settings::BaseController` don't need to define
`#set_body_classes` agein.

app/controllers/settings/base_controller.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class Settings::BaseController < ApplicationController
+  before_action :set_body_classes
+
+  private
+
+  def set_body_classes
+    @body_classes = 'admin'
+  end
+end