Doesn't handle topic names being changed

[zachallaun]

Originally opened by @strugee wtfcarlos/RSVPBot#74:

STR:

1. rsvp init on, say, the topic foobar
2. Edit the topic to, say, foobaz
3. Use a command, e.g. rsvp ping
4. Notice how RSVPBot PMs you that foobaz is not an RSVPBot event