feat(scheduled-messages): Schedule messages / reminders

[jxn-30]

Is your feature request related to a problem? Please describe.
Sometimes you want to remind yourself or other server members on something. A scheduled message would solve that problem

Describe the solution you'd like

### Tasks
- [ ] schedule a message
- [ ] cancel scheduled messages
- [ ] schedule and repeat a message (e.g. each monday at 12 pm CET)
- [ ] cancel a repeated message (only a single one or the whole series)

Describe alternatives you've considered
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Additional context
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