Countdown revamp

Summary:
countdown_timer table named to countdown.
datepoint and related stuff renamed to epoch.
Countdowns now have phids.
Various UI items changed from timer to countdown.

Test Plan: Did run storage upgrade and added some countdowns.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T2624

Differential Revision: https://secure.phabricator.com/D5812