Consolidate all the random code into a random helper that offers:

random::hash()
  random::string()
  random::percent()
  random::int()

So that we don't have lots of different ways to get random values all
over the code.  Follow-on to #1527.

modules/digibug/controllers/digibug.php

@@ -28,7 +28,7 @@ class Digibug_Controller extends Controller {
       $thumb_url = $item->thumb_url(true);
     } else {
       $proxy = ORM::factory("digibug_proxy");
-      $proxy->uuid = md5(mt_rand());
+      $proxy->uuid = random::hash();
       $proxy->item_id = $item->id;
       $proxy->save();
       $full_url = url::abs_site("digibug/print_proxy/full/$proxy->uuid");