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]