Instead of naeveily checking for all reminders due in the next minute,
we now check all reminders in all time zones in the next minutes.
This essentially means checking for reminders due in the next 14 or past
12 hours. We then check for each user who would receive a reminder from
that result if it is actually due in their time zone.
This should prevent issues where users would get the reminder in the
time zone of their server, not in their own.
