Create message reminders

[gnprice]

This is an upcoming feature in Zulip Server 11. It'd be good to add support for it.

This will probably come after support for an older feature with overlapping use cases: "send later", aka "scheduled messages".

References

• Original feature request: feature request: remind me of this message in [time] zulip#1739
• Server/web issue with concrete plan: UI for message reminders zulip#21810
• Server/web PR: Implement remind me feature. zulip#27169
• Server PR that added API endpoint: reminders: Add API endpoint to schedule reminders. zulip#34505
• #api design > Reminde me of message in <time> API #21810