Add application and object level policy controls to Countdown

Summary: Ref T603. Give countdowns proper UI-level policy controls, and an application-level default policy. Put policy information in the header.

Test Plan:
  - Adjusted default policy.
  - Created new countdowns.
  - Edited countdowns.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7322