Previously, when a user was creator and assigned to a task, they would get two reminder notifications for the same task. This was caused by Vikunja first fetching all creators and then all assignees and not removing duplicates from that list.
Related: https://community.vikunja.io/t/duplicate-email-reminders/1505/3
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.
Co-authored-by: kolaente <k@knt.li>
Reviewed-on: vikunja/api#1092
Co-authored-by: konrad <k@knt.li>
Co-committed-by: konrad <k@knt.li>
