R1:6e6ae36dcf93

Add a skeleton for Calendar notifications

Summary:
Ref T7931. I'm going to do this separate from existing infrastructure because:

- events start at different times for different users;
- I like the idea of being able to batch stuff (send one email about several upcoming events);
- triggering on ghost/recurring events is a real complicated mess.

This puts a skeleton in place that finds all the events we need to notify about and writes some silly example bodies to stdout, marking that we notified…