configurable maximum delivery attempts

closes #59

app/views/help/outgoing.html.haml

@@ -18,7 +18,7 @@
         Mail servers can be enabled to send mail from any domain by the administrator.
       %li
         If a message cannot be delivered, the system will not send you a bounce message but dispatch a webhook (if you set one up).
-        If a message delivery fails but can be retried, the system will try #{QueuedMessage::MAX_ATTEMPTS} times to deliver it before giving up.
+        If a message delivery fails but can be retried, the system will try #{Postal.config.general.maximum_delivery_attempts} times to deliver it before giving up.
   .u-margin
     %h2.pageContent__subTitle Sending using SMTP
     %p.pageContent__text