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/base_controller.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module Admin
+  class BaseController < ApplicationController
+    before_action :require_admin!
+
+    layout 'admin'
+  end
+end