Make Countdown Flaggable

Summary: Ref T8888. Cool man, like, more flags.

Test Plan: Flag it, Tag it, and Bag it.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8888

Differential Revision: https://secure.phabricator.com/D13653