Fix admin notification with relative URLs

The test was passing because it was using the rack driver, but on real
browsers it wasn't generating the expected URL.

app/models/admin_notification.rb

@@ -42,7 +42,7 @@ class AdminNotification < ApplicationRecord
     def complete_link_url
       return unless link.present?
 
-      unless self.link[/\Ahttp:\/\//] || self.link[/\Ahttps:\/\//]
+      unless link =~ /\A(http:\/\/|https:\/\/|\/)/
         self.link = "http://#{self.link}"
       end
     end