Since this delayed sending feature is the only thing
settings.MANDRILL_API_KEY is used for, it seems reasonable for that to
be the gate as to whether we actually use Mandrill.
ScheduledJobs with type Email displace the usual mandrill codepaths
in the Zulip Enterprise deploys
* Email-specific helper functions will appear in deliver_email.py
* 0058_auto__add_scheduledjob.py
(imported from commit 8db08d8a279600322acfdbed792dc1a676f7a0ab)