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.

Test Plan:

• 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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7939

Differential Revision: https://secure.phabricator.com/D16368