R1:cdec31914390

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