Add an AdminController base class for Admin controllers

Handles stuff that's shared across admin controllers.

app/controllers/admin_controller.rb

@@ -0,0 +1,11 @@
+# Provides a base class for Admin controllers to subclass
+#
+# Automatically sets the layout and ensures an administrator is logged in
+class AdminController < ApplicationController
+  layout 'admin'
+  before_filter :authenticate_admin!
+
+  def authenticate_admin!
+    return render_404 unless current_user.is_admin?
+  end
+end