Authorize access to admin panel, etc.

Adds a separate admin namespace and base controller, with authorization
by looking up the admin property in the user's LDAP account.

app/controllers/admin/base_controller.rb

@@ -0,0 +1,6 @@
+class Admin::BaseController < ApplicationController
+
+  before_action :authenticate_user!
+  before_action :authorize_admin
+
+end