R1:66975fa51bc3

Implement "trigger clocks" for scheduling events

Summary:
Ref T6881. This will probably make more sense in a couple of diffs, but this is a class that implements scheduling/recurrence rules. Two rules are provided:

- Trigger an event at a specific time (e.g., a meeting reminder notification).
- Trigger an event on the Nth day of every month (e.g., a subscription bill).

At some point, we'll presumably add a rule for T2896 (maybe using the "RRULE" spec) so you can do stuff like "the second to last…