Convert Countdown to EditEngine
Summary: Fixes T10684. Fixes T10520. This primarily implements a date/epoch field, and then does a bunch of standard plumbing.
Test Plan:
- Created countdowns.
- Edited countdowns.
- Used HTTP prefilling.
- Created a countdown ending on "Christmas Morning", etc.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10520, T10684
Differential Revision: https://secure.phabricator.com/D15655