Add Arbitrary Future Date Scheduling

[MrChadMWood]

💡 Idea
Allow users the ability to submit an event to occur for an arbitrary date in the future. More than "3 days" as described here.

The idea is that users would know a date ahead of time. For example: Reminders. Remind yourself to renew something before it expires,

💻 Target components
Server side. I'm not sure if ntfy current handles events in memory or on disk. If in memory, then events longer than 3 days should probably be committed to durable storage until closer.

[binwiederhier]

Done in #1050