Support for in-game events

[lekeno]

Extend the NOTAMs functionality to advertise in-game events like Arena: 51.

• Name of the event
• Date and time
• Repeat pattern
• System / Place
• Description
• PoC / link?

!events command and automatic heads up for upcoming events:
[Events]
Tomorrow, 8:00PM (local) Arena 51: "organized fights in stock ships [...]" in [system]
[...]