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/models/user.rb

@@ -19,4 +19,12 @@ class User < ApplicationRecord
     clear_reset_password_token if valid?
     save
   end
+
+  def is_admin?
+    admin ||= if admin = Devise::LDAP::Adapter.get_ldap_param(self.cn, :admin)
+                !!admin.first
+              else
+                false
+              end
+  end
 end