(chore): temporaily block fiverr for too much spam

Core/Security/ProhibitedDomains.php

@@ -295,5 +295,6 @@ class ProhibitedDomains
         '/amusecandy.com',
         '/360mate.com',
         'rebrand.ly',
+        'fiverr.com', // temporarily until better defence is built
     ];
 }