Decouple RSVPBot from new event announcements

[zachallaun]

RSVPBot announces new events in RSVPs > announce, a thread on Zulip. However, because it's RSVPBot announcing the events, it doesn't really make sense to announce events that can't be easily RSVP'd to. For instance, recurring events (#19) or appointment blocks aren't trivially RSVP'd to.

It would be nice for these kinds of events to be announced on Zulip as well when they're added to the RC calendar. One possible solution could be a "Calendar Bot" that does the announcing in a stream other than RSVPs, and for events that are compatible with RSVPBot, it can post how to initialize them.