Profile.next_nag_date tracks when the next hourly/daily reminder
should be sent. Normally, sendalerts sets this field when
a check goes down, and sendreports clears it out whenever
it is about to send a reminder but realizes all checks are up.

The problem: sendalerts can set next_nag_date to a non-null
value, but it does not clear it out when all checks are up.
This can result in a hourly/daily reminder being sent out
at the wrong time. Specific example, assuming hourly reminders:

13:00: Check A goes down. next_nag_date gets set to 14:00.
13:05: Check A goes up. next_nag_date remains set to 14:00.
13:55: Check B goes down. next_nag_date remains set to 14:00.
14:00: Healthchecks sends a hourly reminder, just 5 minutes
       after Check B going down. It should have sent the reminder
       at 13:55 + 1 hour = 14:55

The fix: sendalerts can now both set and clear the next_nag_date
field. The main changes are in Project.update_next_nag_dates()
and in Profile.update_next_nag_date(). With the fix:

13:00: Check A goes down. next_nag_date gets set to 14:00.
13:05: Check A goes up. next_nag_date gets set to null.
13:55: Check B goes down. next_nag_date gets set to 14:55.
14:55: Healthchecks sends a hourly reminder.

To send notifications, sendalerts calls Flip.send_alerts().
I updated Flip.send_alerts() to be a generator, and to yield
a (channel, error, send_time_in_seconds) triple per sent
notification.

This avoids duplicate calls to Check.get_grace_start() in several places.

Allow `sendalerts` to be run by an external scheduler, on a one-shot
basis, exiting after all alerts have been sent.