CLC-3921 Support updates of events 

https://jira.ets.berkeley.edu/jira/browse/CLC-3921

Supports updates to class events, and does not squash attendees' responses (yes, no, maybe). 
On the other hand, we can always replace the implementation of random_id when/if we feel like it. So yeah, on second thought, a "random_id" helper method might be helpful.