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.

config/routes.rb

@@ -7,7 +7,10 @@ Rails.application.routes.draw do
   get 'welcome', to: 'welcome#index'
   get 'check_your_email', to: 'welcome#check_your_email'
 
-  get 'ldap_users', to: 'ldap_users#index'
+  namespace :admin do
+    root to: 'dashboard#index'
+    get 'ldap_users', to: 'ldap_users#index'
+  end
 
   # Letter Opener (open "sent" emails in dev and staging)
   if Rails.env.match(/staging|development/)