Moved env vars to common global config files, so that theycan be used in non-docker contexts too

config/autoload/delete_short_urls.global.php

@@ -4,11 +4,15 @@ declare(strict_types=1);
 
 namespace Shlinkio\Shlink;
 
+use function Shlinkio\Shlink\Common\env;
+
+use const Shlinkio\Shlink\Core\DEFAULT_DELETE_SHORT_URL_THRESHOLD;
+
 return [
 
     'delete_short_urls' => [
-        'visits_threshold' => 15,
         'check_visits_threshold' => true,
+        'visits_threshold' => (int) env('DELETE_SHORT_URL_THRESHOLD', DEFAULT_DELETE_SHORT_URL_THRESHOLD),
     ],
 
 ];