Initialize an event automatically when an event-modifying command is used for the first time.

[zachallaun]

Originally opened by @kokeshii wtfcarlos/RSVPBot#47:

When a user does rsvp yes and there's no event associated with the Zulip thread, we should automatically and silently initialize an event in that thread. This helps to reduce spam, cognitive load, and lowers the learning curve of the bot a little bit.

I think the equivalent given the RC calendar integration would be to PM the person doing the rsvp yes with a link to create the event on RC (which has the effect of auto-RSVPing them as well).