[manual] digest: add the cron job that will trigger digest emails.

Manual deployment instructions:

On staging, do a puppet apply.

No action needs to be taken for the prod deploy.

(imported from commit 0f6e5ab22aaeacfcc69d57de12f2bb6fac6f0635)

puppet/zulip_internal/files/cron.d/send-digest-emails

@@ -0,0 +1,4 @@
+MAILTO=root
+
+# Send digest emails once a day.
+0 11 * * *   zulip cd /home/zulip/deployments/current && python manage.py enqueue_digest_emails

puppet/zulip_internal/manifests/staging_app_frontend.pp

@@ -38,6 +38,13 @@ class zulip_internal::staging_app_frontend {
     mode => 644,
     source => "puppet:///modules/zulip_internal/cron.d/clearsessions",
   }
+  file { "/etc/cron.d/send-digest-emails":
+    ensure => file,
+    owner  => "root",
+    group  => "root",
+    mode => 644,
+    source => "puppet:///modules/zulip_internal/cron.d/send-digest-emails",
+  }
   # Staging has our Apple Push Notifications Service private key at
   # /etc/ssl/django-private/apns-dev.pem
 }