retention: Add a daily cron job.

This will run archive_messages management command at 6am every day, 1
hour after soft_deactivate_users (which runs at 5am).

puppet/zulip/files/cron.d/archive-messages

@@ -0,0 +1,7 @@
+SHELL=/bin/bash
+PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
+USER=zulip
+
+# Daily cron job for retention policy - archives existing messages based on the policy
+# and cleans expired entries from the archive.
+0 6 * * * zulip /home/zulip/deployments/current/manage.py archive_messages >/dev/null

puppet/zulip/manifests/app_frontend_once.pp

@@ -34,6 +34,14 @@ class zulip::app_frontend_once {
     source => 'puppet:///modules/zulip/cron.d/soft-deactivate-users',
   }
 
+  file { '/etc/cron.d/archive-messages':
+    ensure => file,
+    owner  => 'root',
+    group  => 'root',
+    mode   => '0644',
+    source => 'puppet:///modules/zulip/cron.d/archive-messages',
+  }
+
   file { '/etc/cron.d/calculate-first-visible-message-id':
     ensure => absent,
   }