Bot does not handle holidays or missing events

[fredlawl]

The bot should have a (internal/external/cache?) list of known U.S. holidays, and if an event is going to happen the day of the holiday, don't post a message for that week.

Same for missing events, if we know there's no upcoming event for the week, don't announce. In cases we don't know if there's an event or not, such as meetup isn't hooked up/enabled, assume there is an event--but respect the holiday rule.

TODO: https://github.com/Open-SGF/discord-bot/blob/main/bot/bot.go#L109

Aside: This is an interesting one because maybe we pull a list from an outside source on bot startup, and cache that to a file we can read from weekly.

EDIT: Added detail for missing meetup events