Be smarter about getting random albums with art

Just ask the database for what we want instead of asking for more
results, sifting through them, and hoping that we get enough
qualifying results to fulfill the request.

Also drop Random::album(), since it's duplicate code and it makes more
sense for this to live in Album.

server/random.ajax.php

@@ -27,7 +27,7 @@ if (!defined('AJAX_INCLUDE')) { exit; }
 
 switch ($_REQUEST['action']) {
         case 'album':
-                $album_id = Random::album();
+                $album_id = Album::get_random();
 
         // If we don't get anything stop
         if (!$album_id) { $results['rfc3514'] = '0x1'; break; }