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

spec/controllers/admin/settings_controller_spec.rb

@@ -0,0 +1,14 @@
+require 'rails_helper'
+
+RSpec.describe Admin::SettingsController, type: :controller do
+  describe 'GET #index' do
+    before do
+      sign_in Fabricate(:user, admin: true), scope: :user
+    end
+
+    it 'returns http success' do
+      get :index
+      expect(response).to have_http_status(:success)
+    end
+  end
+end