Certificate file name extensions unified (#354)

* Fix for default smtp certificate name

This fixes the defaut smtp certificate file name from smtp.crt to smtp.cert as it is used throughout the postal code.

resource/nginx.cfg

@@ -10,7 +10,7 @@ server {
     listen 0.0.0.0:443 ssl;
     root /opt/postal/app/public;
     server_name postal.yourdomain.com;
-    ssl_certificate      ssl/postal.crt;
+    ssl_certificate      ssl/postal.cert;
     ssl_certificate_key  ssl/postal.key;
 
     # Generate using: openssl dhparam 4096 -out /etc/ssl/dhparam.pem