Admin base controller (#1465)

* Add Admin::BaseController to wrap admin area

Extracts the setting of the `admin` layout and verifying that users are admins
to a common base class for the admin/ controllers.

* Add basic coverage for admin/reports and admin/settings controllers

app/controllers/admin/pubsubhubbub_controller.rb

@@ -1,11 +1,9 @@
 # frozen_string_literal: true
 
-class Admin::PubsubhubbubController < ApplicationController
-  before_action :require_admin!
-
-  layout 'admin'
-
-  def index
-    @subscriptions = Subscription.order('id desc').includes(:account).paginate(page: params[:page], per_page: 40)
+module Admin
+  class PubsubhubbubController < BaseController
+    def index
+      @subscriptions = Subscription.order('id desc').includes(:account).paginate(page: params[:page], per_page: 40)
+    end
   end
 end