Move email digest triggering to default zulip config.

Previously, even though the Zulip digest emails were documented in the
settings, the cron job to run the script that actually sends the daily
digest emails wasn't included in the non-zulip.com part of the Zulip
production distribution.  The overall consequence is that digest
emails didn't work for non-zulip.com users.  This fixes that issue by
moving that cron job into the zulip manifests.

[commit message details expanded by tabbott]

puppet/zulip/manifests/app_frontend.pp

@@ -18,6 +18,15 @@ class zulip::app_frontend {
     notify => Service["nginx"],
   }
 
+  # Trigger daily digest e-mails
+  file { "/etc/cron.d/send-digest-emails":
+    ensure => file,
+    owner  => "root",
+    group  => "root",
+    mode => 644,
+    source => "puppet:///modules/zulip/cron.d/send-digest-emails",
+  }
+
   # Restart the server regularly to avoid potential memory leak problems.
   file { "/etc/cron.d/restart-zulip":
     ensure => file,

puppet/zulip_internal/manifests/prod_app_frontend.pp

@@ -94,13 +94,6 @@ class zulip_internal::prod_app_frontend {
     mode => 644,
     source => "puppet:///modules/zulip_internal/sparkle/sso/win/sparkle-changelog.html",
   }
-  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",
-  }
 
   # Prod has our Apple Push Notifications Service private key at
   # /etc/ssl/django-private/apns-dist.pem