Countdown revamp 2

Summary:
Now has policy support (not really)
Now uses CountdownQuery
Now has handles
Now uses common way for remarkup
Remarkup still looks terrible

Test Plan: Added countdowns, edited countdowns. Did even embed some. Couldn't break it

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T2624

Differential Revision: https://secure.phabricator.com/D6012