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