Add basic Herald support to Calendar
Summary: Fixes T7939. This doesn't get too fancy, but allows you to write Herald rules against Calendar events.
- Wrote an "add red flag to events with party in the name" rule.
- Created a "mundane meeting", didn't get flagged.
- Created a "cool party", got flagged.
- Ran rules from the Herald test console.
Reviewed By: chad
Maniphest Tasks: T7939
Differential Revision: https://secure.phabricator.com/D16368